Background

Programmed cell death protein 1 (PD-1) is a receptor predominantly expressed on activated CD8+ T cells that inhibits tumor-specific T-cell responses in cancer, and specific monoclonal antibodies (mAbs) blocking the PD-1 receptor can relieve PD-1/programmed cell death protein ligand 1 (PD-L1) pathway mediated immune suppression.1 Anti-PD-1 mAb immunotherapy has revolutionized cancer treatment by effectively treating a diverse range of cancer types.2 Most clinically approved anti-PD-1 mAbs were engineered using the IgG4 framework3 4 as IgG4 does not facilitate classical antibody-dependent effector functions,5 6 which avoids autoimmune injury to the target immune cells. The natural IgG4 structure is instable and it can cleave into half molecules and rehybridize, known as the Fab-arm exchange (FAE). Therefore, IgG4 isotype with the S228P mutation IgG4 isotype (IgG4 S228P) is often used in anti-PD-1 mAbs to stabilize the molecule but still retaining high affinity to FcγRI and binding to FcγRIIb such as nivolumab and pembrolizumab.7 8 However, the improvement is insufficient as IgG4 induces immune inhibition through various mechanisms, particularly with its Fc. The known mechanisms including but not limited to the obstruction of antibody-mediated effector functions through competitive binding antigens or Fc receptors (FcRs),5 6 9 and inducing immunosuppressive M2 macrophage phenotypes through FcR interactions.10 11 The S228P mutated anti-PD-1 mAbs were also found to retained similar characteristics of IgG4. Previous studies found that binding interactions between IgG4 Fc and FcγR could lead to nivolumab (an IgG4 S228P anti-PD-1 mAb) transfer from CD8+ T cells to macrophage,12 induce M2-like macrophage phenotype and tumor progression.13 Fc:FcγR interactions activated by IgG4 anti-PD-1 mAbs also led to depletion of activated CD8+ T cells and abrogate their therapeutic activity while its engineered Fc-null counterpart enhanced endogenous CD8+ T cell expansion.14 15 These mechanisms are speculated to be associated with therapeutic resistance and the occurrence of hyperprogressive disease (HPD) in anti-PD-1 mAb cancer immunotherapy, with an incidence ranging from 5.9% to 43.1%.4 16

The Fc–Fc interactions are another property unique to IgG4, where the Fc fragment of IgG4 binds to the Fc fragment of other immobilized IgGs and reacts more strongly with IgG1.17 Our recent investigations found that non-specific IgG4 can inhibit antibody-dependent cellular cytotoxicity (ADCC), antibody-dependent cellular phagocytosis (ADCP), and complement-dependent cytotoxicity (CDC) mediated by IgG1 antitumor antibodies through Fc–Fc interactions. This interaction can be augmented by elevated levels of glutathione (GSH) in the tumor microenvironment.18 19 As IgG20 and GSH21 were found abundantly distributed in various tumors, the Fc–Fc interactions between IgG4 and immobilized IgG may be further facilitated in the cancer microenvironment. Notably, the S228P mutation has minimal impact on the Fc–Fc interactions of IgG4, but the R409K mutation at the Fc terminal is crucial for such reaction.17 We found that the nivolumab still retained Fc–Fc interactions19 like wildtype IgG4 which could lead to antibody blocking and off-target effects, possibly causing resistance to anti-PD-1 mAb therapy. In clinical practice, there is ongoing research on combining IgG4 S228P anti-PD-1 mAb with other IgG1 antibodies, such as nivolumab with trastuzumab,22 pembrolizumab with rituximab,23 nivolumab with cetuximab,24 and nivolumab with ipilimumab.25 While some studies showed promising efficacy, it is important to further evaluate the risks of dual antibody interaction between IgG4 and IgG1 due to their strong tendency for Fc–Fc reaction. To mitigate these risks, newer versions of PD-1 antibodies like penpulimab26 and tislelizumab27 have incorporated optimization strategies such as framework replacements with Fc-null IgG1 and IgG4 S228P+R409K isotype. By implementing Fc-null modifications, interactions between Fc of anti-PD-1 mAb and FcR of immune cells can be eliminated, resulting in the absence of ADCC, ADCP, and antibody-dependent cytokine release effects.26 The Fc-null IgG1 backbone offers a more structurally stable framework and solution stability28 without the Fc–Fc interactions or FAE. Therefore, Fc-null IgG1 anti-PD-1 mAb has the potential to improve the efficacy and safety of PD-1 blockade combination therapies. In this study, we analyzed the structure and function of IgG4 S228P and Fc-null IgG1 anti-PD-1 mAb and evaluated how their frameworks could affect the safety and effectiveness of dual-antibody combination therapies through Fc–Fc interactions. The study also examined how IgG4 and IgG1 affect the growth and suppression of immune cells commonly found in cancer environments.

MethodsKey resources

Online supplemental table S1 provides a comprehensive list of the essential resources used in this study, including antibodies, chemicals, detection kits, cell lines, and software.

Healthy volunteers and tissue samples

Healthy volunteers provided blood samples, which were used to isolate and differentiate various immune cell types. Cancer tissues from the lung, esophagus, and colon (six samples each) were collected from the Shantou University Affiliated Tumor Hospital. The levels of IgG, IgG1, and GSH were assessed.

Immunohistochemistry and antigen preabsorption tests

Cancer tissues from the esophagus, lungs, and colon were fixed in formalin, embedded in paraffin, and cut into 4 µm thick sections. The sections were then dried at 65°C for 3 hours. Deparaffinization was done by incubating the sections in xylene three times for 10 min each, followed by rehydration using graded alcohol solutions. Immunohistochemistry (IHC) was performed using anti-IgG, IgG1, and GSH antibodies (Abcam) to study the expression and location of IgG, IgG1, and GSH in the tumor microenvironment. Confirm the specificity of the staining, we performed antigen preabsorption tests using antibodies that were preincubated with specific antigens. Specifically, 100 µL of antibody solutions containing a gradient concentration of IgG (Solarbio), IgG1 (Athens Research), and GSH (Sigma-Aldrich) antigens were preincubated at 4°C for 3 hours. These preincubated solutions were then used as primary antibodies for the staining process.

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Coomassie staining

To test antibody tolerance to GSH and detect IgG half-molecule, both reduced and non-reduced sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis (PAGE) and Coomassie blue staining were used. The samples included wildtype IgG1 (IgG1wt) and IgG4 (IgG4wt), various versions of IgG1 and IgG4 anti-PD-1 mAbs including nivolumab (IgG4 S228P), pembrolizumab (IgG4 S228P), and penpulimab (AK 105, Fc-null IgG1), tislelizumab (IgG4 S228P+R409K), as well as antitumor IgG1 antibodies trastuzumab, rituximab, and cetuximab. Detailed protocols and specific experimental conditions are shown in online supplemental information.

Papain digestion

The Fc fragments of IgG4wt, nivolumab, and penpulimab were obtained using the Pierce Fab Preparation Kit (Thermo Scientific) and labeled with biotin for Fc–Fc binding tests. Additional information can be found in online supplemental information.

Protein labeling

Biotin labeling was performed following the instructions provided by Biotin Protein Labeling Kit (Ana spec) and described in online supplemental methods. IgG1wt, IgG4wt, penpulimab, nivolumab, tislelizumab, rituximab, trastuzumab, inetetamab, bevacizumab, pembrolizumab, sintilimab, and Fc fragments of IgG4wt, penpulimab, and nivolumab were all biotin-labeled as primary antibodies for Western blot, ELISA tests, or immunocytochemistry.

Western blot

Various types of proteins including subtypes of standard IgG proteins, mAbs, and standard IgGs from human, mouse, rabbit, and goat were immobilized on nitrocellulose membranes using electrophoretic transfer as target proteins. Biotin-labeled IgG4wt and IgG1wt, along with IgG4 and IgG1 backbone mAbs, were used as primary antibodies to assess their binding interactions with the proteins on the membrane. To confirm the Fc–Fc interactions, IgG Fc fragments were obtained by digesting IgGs with papain. The Fc fragments were then labeled with biotin and employed as primary antibodies to validate the presence of Fc–Fc interactions. Detailed methods can be found in online supplemental information.

ELISA and surface plasmon resonance assays

To further analyze the characteristics of Fc–Fc interactions and determine the kinetics, ELISA and biacore analysis (surface plasmon resonance, SPR) measurements were conducted. Briefly, IgG1wt was immobilized on high-binding microplates or biacore sensor chips as the target. Serial dilutions of IgG1wt, IgG4wt, penpulimab, nivolumab, or tislelizumab were added to the ELISA wells or flowed through the sensor chip to assess binding. Quantitative data was collected using a microplate reader (BioTek, EPOCH 2) or a Biacore T200 SPR system. Detailed methods can be found in online supplemental information.

Cell lines

THP-1, Raji, and BT-474 cell lines were obtained from Procell Life Science & Technology Co., Ltd. The THP-1 cells were cultivated in complete medium (Procell Life Science) containing RPMI 1640 media supplemented with 10% fetal bovine serum, 0.05 mM β-mercaptoethanol, and 100 U/mL penicillin-streptomycin. Raji cells were cultured in RPMI 1640 media supplemented with 10% fetal bovine serum and 100 U/mL penicillin-streptomycin (all Gibco). BT-474 cells were cultured in complete medium (Procell Life Science) consisting of RPMI 1640 media supplemented with 10 µg/mL insulin, 2 mM L-glutamine, 20% fetal bovine serum, and 100 U/mL penicillin-streptomycin. All cells were cultured under standard conditions of 37°C and 5% CO2. The cell lines were cultured specifically for ADCC, ADCP, and CDC cell model tests.

Immunocytochemistry and immunofluorescence

BT-474 and Raji cells were attached to glass slides, and trastuzumab and rituximab were bound to these cells through specific interactions with their corresponding cell antigens. Biotin-labeled IgG1wt and IgG4wt, along with IgG1 and IgG4 backbone anti-PD-1 mAbs, were employed to assess their binding interactions with the cells or the cell-bound trastuzumab or rituximab. More details can be found in online supplemental information.

Immune cells isolation and preparation

Immune cells (CD8+ T cells, natural killer (NK) cells, and monocytes) were isolated from healthy donors’ peripheral blood using immune cell separation kits. THP-1 cells were stimulated and differentiated to create M1 macrophages. These immune cells and M1 macrophages were then cultured and prepared for in vitro cellular tests. Detailed procedures can be found in online supplemental information.

ADCC, ADCP, and CDC assays

Two cellular models (Raji and BT-474) were used to study the effects of Fc-null IgG1 and IgG4 S228P anti-PD-1 antibodies on immune responses mediated by trastuzumab and rituximab. Tumor cells were treated with antibodies, washed to remove excess antibodies, and then tested for killing ability with immune cells or serum. IgG1wt and IgG4wt were used as isotype controls. Time-lapse fluorescence microphotography was conducted for multiple wells in ADCP experiments lasting 12 hours. Further details can be found in online supplemental information.

Analysis of cytokines

The supernatant samples obtained from BT-474 ADCC cell experiments were divided into aliquots and immediately frozen at −80°C following centrifugation. The analysis of cytokines was conducted using a multiplex kit (Merck Millipore, HSTCMAG-28SK) and the Luminex MAGPIX System (Luminex Corporation). The cohort samples were assessed for the presence of various cytokines, chemokines, and growth factors, including interleukin (IL)-1β, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12p70, IL-13, IL-17A, IL-21, IL-23, tumor necrosis factor-α (TNF-α), granulocyte-macrophage colony-stimulating factor, interferon-γ (IFN-γ), macrophage inflammatory protein-1 beta, and macrophage inflammatory protein-3 alpha.

Tests of IgG subtypes on immune cells

We examined how different IgG subtypes affect cytokine secretion and protein expression in CD8+ T cells, monocytes, and NK cells. This was done by culturing immune cells with different subtypes of IgG and using techniques including ELISA, flow cytometry, and RT-PCR to assess cytokine secretion, protein expression, and mRNA profiles. An FcR blockade test was also conducted to confirm the binding of IgG4 to FcR and its interaction with CD8+ T cells and NK cells. More information can be found in online supplemental methods.

Animal models

4T1 Breast cancer and CT26 colon cancer tumor bearing models were established in wildtype BALB/c mice (n=45, female) or humanized PD-1 BALB/c-hPD1 mice (n=20, female). Experimental mice were obtained from Vital River Technology (Beijing, China) and GemPharmatech (Nanjing, China). Mice aged between 8 and 10 weeks, weighing 20±2 g, were selected for all experimental procedures and maintained in specific pathogen-free conditions. The mice were assigned to separate treatment groups by random-numbers table, where they received either IgG1wt, IgG4wt, penpulimab, or nivolumab, with the phosphate-buffered saline (PBS) group designated as the control. Proteins were dissolved in PBS at 1 mg/mL and administered peritumorally at a dose of 100 µg every 5 days for four injections. Cancer models were induced by injecting 4T1 (n=5/group, 1×105 cells for wild-type and 5×104 cells for BALB/c-hPD1 model per mouse) or CT26 (n=4/group, 5×104 cells per mouse) cells subcutaneously under the left forearm, with tumors becoming measurable within 5–7 days after the initial protein injection. The longest diameter (a) and the shortest diameter (b) of the tumor were measured every 3 days using a caliper, and the volume (V) was then calculated using the formula: V=1/2×a×b2. If the tumor volume exceeds 2000 mm3 or the longest diameter exceeds 2 cm, mice would be humanely euthanized. After 3 weeks, the mice were anesthetized and euthanized by intraperitoneal overdose of 0.1% sodium pentobarbital. The tumor tissues were weighed and fixed in 10% formalin solution for 24 hours, followed by dehydration and embedding in paraffin. The tissue blocks were cut into 4 µm sections and stained with antibodies for CD3, CD4, CD8, CD31, F4/80, and CD163. T cell and macrophage infiltration, as well as tumor angiogenesis, were quantitatively analyzed by capturing five high-power views of the slides and processing the images with ImageJ software.

Statistical analysis

Statistical analyses were performed using GraphPad Prism software V.9 (GraphPad Software, La Jolla, CA, USA). Results are shown as mean±SD unless stated otherwise. Statistical significance is shown as *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001, and ns=not statistically significant. Statistical significance was determined using a one-way analysis of variance with Bonferroni’s correction for multiple comparisons.

ResultsTumor microenvironment creates a conducive environment for the Fc–Fc interactions of IgG4

IHC analysis of esophageal, lung, and colon cancer tissues (n=6 per type) showed significant presence and distribution of IgG, IgG1, and GSH within tumor tissues (figure 1A). Stronger staining in cancer nests compared with stroma suggests these antibodies may be tumor-specific and able to bind to tumor cells. Antigen preabsorption experiments confirmed the specificity of immunohistochemical staining (figure 1B). These findings showed that the tumor microenvironment is conducive for the Fc–Fc interactions of IgG4 or IgG4-type mAbs. This highlights the potential impact of IgG4-mediated immunosuppressive effects in cancer immunotherapy as the abundance of IgG, IgG1, and GSH in the tumor microenvironment creates a favorable setting for the Fc–Fc interactions involves IgG4 or IgG4 S228P anti-PD-1 mAbs.

Figure 1

Immunohistochemistry staining results of IgG, IgG1, and glutathione (GSH) in various cancer tissues. (A) Immunohistochemistry staining showed significant presence of IgG, IgG1, and GSH in esophageal, lung, and colon cancer tissues (n=6/type). Scale bar=200 µm. (B) The specificity of IgG, IgG1, and GSH immunohistochemical staining in tumor tissues was confirmed by antigen preabsorption tests. Higher concentrations of specific antigens led to less staining on the tumor tissue. Scale bar=60 µm.

IgG4 and clinical therapeutic IgG4 S228P anti-PD-1 mAb bind to immobilized IgGs through Fc–Fc interactions

Figure 2 shows that both wildtype IgG4 and S228P mutated IgG4 anti-PD-1 mAb can bind to immobilized IgGs through Fc–Fc interactions. The Western blot analysis in figure 2A demonstrates that biotin-labeled IgG4wt, IgG4 S228P variant penpulimab, and nivolumab bind to immobilized IgGs, including IgG1-type antibodies like trastuzumab and rituximab. The strength of Fc–Fc interactions between IgG4 and IgG1 is stronger than between IgG4 and IgG4. Figure 2B provides additional evidence supporting the ability of IgG4wt and nivolumab to interact with four different IgG subtypes and IgG derived from various species (such as human, mouse, rabbit, and goat). This finding suggests that the Fc–Fc interactions of IgG4 demonstrates a trait that is evolutionarily conserved.

Figure 2

IgG4 and IgG4 S228P anti-programmed cell-death 1 monoclonal antibody (mAb) bind to immobilized IgG through Fc–Fc interactions. (A) The Western blots demonstrate that biotin-labeled IgG4wt, nivolumab and IgG4 S228P variant penpulimab demonstrate binding capability to immobilized IgG on the membranes (1 µg/well). Biotin-labeled IgG1wt, penpulimab (Fc-null IgG1), and Fc functional IgG1 variant penpulimab (similar to IgG1wt) do not exhibit a similar binding reaction. (B) IgG4wt and nivolumab boud to all IgG subtypes and cross-species IgGs (sample loading 2 µg/well), with stronger binding to IgG1 than IgG4. Penpulimab does not show the same binding pattern. (C) The binding mode involves Fc–Fc interactions between IgG4wt, nivolumab, and immobilized IgG on the membrane. (D) Increasing GSH concentration from 0 to 3 mM enhanced Fc–Fc interactions between IgG4wt/nivolumab and immobilized IgG1 or IgG4 on the membrane, with stronger reactions observed with IgG1. (E) Sintilimab and pembrolizumab, both IgG4 S228P mutated variants, showe similar Fc–Fc interactions to IgG4wt, while IgG1wt, inetetamab and bevacizumab (both IgG1 mAb) did not bind. (F) Left, IgG1wt was immobilized on ELISA plate wells at a concentration of 10 µg/mL in 100 µL PBS. Various concentrations of biotin-labeled IgG1wt, IgG4wt, penpulimab, nivolumab, and tislelizumab were introduced to assess their binding affinity to the immobilized IgG1wt. Right, biotin-labeled penpulimab and tislelizumab exhibit a lack of Fc–Fc interactions like IgG4wt and nivolumab (sample loading 2 µg/well). (G) IgG1wt was immobilized on CM5 biocore sensor chips at a coupling level of 8000RU. IgG4wt and nivolumab at concentrations ranging from 312 to 5000 nM at a flow rate of 2 µL/min on the chips. IgG1wt was set as the control. T25°C, association time 240 s and dissociation time 240 s. GSH, glutathione; Nivo, nivolumab; Pen, penpulimab; Ritu, rituximab; Tisle, tislelizumab; Trastu, trastuzumab.

Figure 2C illustrates the papain digestion assays and Western blot results that validate the mechanism by which IgG4 binds to immobilized IgG. Both IgG4wt and nivolumab demonstrate the ability to bind to the immobilized IgG Fc fragment through Fc–Fc interactions. Figure 2D shows the Fc–Fc interactions under varying concentrations of GSH to simulate elevated GSH levels within the tumor microenvironment. The findings indicate a direct relationship between the strength of Fc–Fc interactions involving IgG4wt, nivolumab, and immobilized IgG on the membrane, and the rising levels of GSH. Furthermore, half molecules were observed in IgG4wt and IgG4 S228P PD-1 mAbs in the presence of gradient concentration of GSH, whereas IgG1wt and IgG1 type antibodies were more stable under the same conditions (online supplemental figure S1). Experiments were also conducted on the other IgG1 (inetetamab, bevacizumab) or IgG4 S228P (sintilimab, pembrolizumab) mAbs. Results indicated that the IgG4 antibody displayed similarities to IgG4wt and demonstrated the capacity to produce Fc–Fc interactions, while the IgG1 antibodies did not exhibit these characteristics, as illustrated in figure 2E. The Fc–Fc interaction pattern was also examined under non-denatured conditions using ELISA experiments. The results in figure 2F show that IgG4wt and nivolumab have comparable reaction capabilities, with reactions increasing in intensity as the dosage increases. Notably, both penpulimab and tislelizumab exhibit a lack of Fc–Fc interactions as seen for IgG4wt indicating that these two antibodies might be similar in eliminating the Fc–Fc interactions. Subsequent analysis of the kinetic data of Fc–Fc interactions between IgG4wt and nivolumab to immobilized IgG1wt was conducted through SPR testing (figure 2G). The estimated apparent dissociation constants were 0.63 µM for IgG4 and 0.81 µM for nivolumab (table 1).

Table 1

Apparent dissociation constants of IgG4wt and nivolumab to immobilized IgG1wt, measured with surface plasmon resonance

IgG4 and IgG4 S228P anti-PD-1 mAb can bind to cancer cells through the Fc–Fc interactions with tumor-targeting antibody

Based on previous findings, we conducted experiments using immunocytochemistry and immunofluorescence to study the potential binding of IgG4 to IgG1 antibodies on tumor cells through Fc–Fc interactions (figure 3A). The expression of HER-2 antigens in breast cancer cells (BT-474) or CD20 antigens in lymphoma cells (Raji) were confirmed with mouse anti-human HER-2 or CD20 antibodies. Biotin-labeled trastuzumab (anti-HER-2 IgG1) and rituximab (anti-CD20 IgG1) showed specific binding to tumor antigens while biotin-labeled IgG1wt, IgG4wt, penpulimab, and nivolumab did not bind. Immunocytochemistry and immunofluorescence confirmed binding of IgG4wt and nivolumab to tumor cells (BT-474 and Raji) after these cell pre-incubation with trastuzumab or rituximab. These results indicate that IgG4 antibodies interact with IgG1 antibodies on tumor cells through Fc–Fc interactions, blocking antitumor antibodies. Conversely, biotin-labeled IgG1wt and penpulimab do not exhibit comparable binding to the tumor cells (figure 3B,C).

Figure 3

The binding of IgG4wt and IgG4 S228P anti-programmed cell-death 1 monoclonal antibody to IgG1 antibodies immobilized on cancer cells. (A) Schematic representation of the immunocytochemistry and immunofluorescence experiments. The presence of HER-2 or CD20 antigens on cancer cell lines (BT-474/Raji) was confirmed using mouse anti-human antibodies. Cancer cells were fixed on slides and incubated with trastuzumab or rituximab to immobilize the antibodies. Biotin-labeled IgG4wt, IgG1wt, penpulimab, and nivolumab were tested for binding to the immobilized IgG1 on the cells. (B) Immunocytochemistry results show Fc–Fc binding reaction between biotin-labeled IgG4wt/nivolumab and trastuzumab immobilized on BT-474 cells. (C) Similar immunofluorescence results seen with rituximab on Raji cells. Positive binding indicated by brown staining or green fluorescence. Scale bar=60 µm. Ritu, rituximab; Trastu, trastuzumab.

IgG4 S228P anti-PD-1 mAb attenuates the tumor killing effects of tumor-targeting antibodies in vitro

In cell experiments, the effects of IgG1 and IgG4 subtype anti-PD-1 mAbs on trastuzumab and rituximab-mediated ADCC, ADCP, and CDC immune responses were investigated using BT-474 and Raji cell models. Results showed that IgG4wt and nivolumab inhibited the ADCC and ADCP effects mediated by trastuzumab and rituximab, whereas no attenuation was observed in penpulimab groups (figure 4A,B). The ADCC tests were extended to include various other IgG4 and IgG1-type antibodies and we found that all IgG4 S228P anti-PD-1 mAb, except tislelizumab (IgG4 S228P+R409K), inhibited the ADCC effect of trastuzumab, while IgG1-type antibodies like penpulimab, bevacizumab did not have the same effect (online supplemental figure S2). The CDC tests showed that trastuzumab alone did not elicit complement activation, except when combined with pertuzumab, which is consistent with previous reports.29 In a separate experiment, we observed the inhibitory effects of IgG4wt on the trastuzumab plus pertuzumab-mediated CDC effect and IgG1wt not (online supplemental figure S2). In rituximab-mediated CDC tests, only the IgG4wt group showed reduced activity, while the other groups did not. In ADCP experiments, a time-lapse imaging technique using multiple wells and fluorescence microscopy was employed to visually capture the dynamic process of cellular phagocytosis. The recordings documented the phagocytic activity of BT-474 cell models over a 12-hour period for each subgroup. Analysis of the results revealed that in the IgG4wt and nivolumab groups, macrophages had reduced ability to engulf tumor cells, allowing tumor cells to grow and adhere well (online supplemental video 1). Phagocytosis was measured by quantifying overlapping 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (Dil)+/carboxyfluorescein diacetate succinimidyl ester (CFSE)+ cells using fluorescence microscopy and flow cytometry. Dil labeled macrophages and CFSE labeled tumor cells were observed as red and green fluorescent cells under fluorescence microscopy. (online supplemental figure S2).

Figure 4

IgG4 S228P anti-programmed cell-death 1 monoclonal antibody impedes classical immune effect mediated by antitumor antibodies and hinders cytokine secretion in vitro. (A–B) Left, BT474 or Raji cells were seeded at a density of 1×104 cells/well and pretreated with various antibodies. ADCC effect was induced using trastuzumab (2 µg/mL) or rituximab (1 µg/mL). Peripheral blood mononuclear cell-derived natural killer cells (2×104 cells/well) were added to the well and cocultured with tumor cells for 2 hours at 37°C. IgG4wt and nivolumab were found to reduce the ADCC effect mediated by the IgG1 antibodies; middle, M1 macrophages were labeled with Dil, while BT474 breast cancer cells or Raji lymphoma cells were labeled with CFSE. Phagocytosis was mediated with trastuzumab or rituximab at a 1:1 effector-target ratio. The ADCP effect was weakened by IgG4wt and nivolumab; right, Trastuzumab did not activate complement or cause CDC on BT-474 cells. Rituximab activated complement and damaged Raji cells. IgG4wt reduced CDC effect of rituximab, while IgG1wt, penpulimab, and nivolumab did not. (C) The Luminex test results showed that IgG4wt and nivolumab inhibited cytokine release in the ADCC experiment with the BT-474 model. Trastuzumab and trastuzumab plus penpulimab had similar effects on cytokine release. Nivo, nivolumab; Pen, penpulimab; Ritu, rituximab; Trastu, trastuzumab; HI serum, heat-inactivated serum; ADCC, antibody-dependent cellular cytotoxicity; ADCP; antibody-dependent cellular phagocytosis; CDC, complement-dependent cytotoxicity; CFSE, carboxyfluorescein diacetate succinimidyl ester; Dil, 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate; GM-CSF, granulocyte-macrophage colony-stimulating factor; TNFα, tumor necrosis factor α;IFN-γ, interferon γ; IL, interleukin; MIP, macrophage inflammatory protein.

Analysis of cytokines in cell culture supernatants of ADCC in the BT-474 model showed that both IgG4wt and nivolumab inhibited cytokine release in ADCC mediated by trastuzumab. Penpulimab treated groups had similar cytokine release patterns to trastuzumab treated groups (figure 4C), suggesting that IgG4 S228P anti-PD-1 mAb may interfere with immune responses mediated by IgG1 antibodies through Fc–Fc interactions, diminishing their effectiveness in combine immunotherapy.

IgG4 but not IgG1, inhibited the viability and cell function of immune cells in vitro

Numerous antibodies targeting multiple antigens are generated using IgG1 or IgG4 frameworks. The impact of varying concentrations of these IgG subtypes on immune cells was investigated in this study, revealing divergent effects when cocultured with immune cells. Figure 5A shows the effects of four IgG subtypes on activated CD8+ T cells. IgG4 decreased cell viability in a dose-dependent manner after 2 days, while the other subtypes did not. Coculturing with 0.5 mg/mL IgG4 increased CTLA4 and PD-1 mRNA expression in CD8+ T cells. Higher concentrations of IgG1, IgG3, and IgG4 hindered Granzyme B secretion, while IgG4, IgG1, and IVIG inhibited IFN-γ secretion compared with control group. TNFα secretion was significantly inhibited by higher concentrations of IgG4. IgG4 has a stronger inhibitory effect on cytokine secretion by activated CD8+ T lymphocytes compared with other IgG subtypes. IgG4 exerts such immunosuppressive effects by binding to FcRs, and the binding reaction can be inhibited by FcR blocker (figure 5B). Flow cytometry confirmed the purity of CD8+ T cells, and activated cells treated with antibodies showed irregular shapes and clustered together (online supplemental figure S3A,B). Similar tests on NK cells showed that IgG4 at 0.1 mg/mL also significantly inhibited NK cell activity, secretion of Granzyme B and IFN-γ, and downregulated the expression of CD69, CD314, and CD226 on NK cells (online supplemental figure S3C,D).

Figure 5

Comparison of the direct effects of different IgG subtype proteins on CD8+ T cells and monocytes. (A) The upper panel, the cell viability of activated CD8+ T cells treated with lower (0.05 mg/mL) and higher concentrations (0.5 mg/mL) of IgG subtypes or IVIG was assessed using CCK-8 kits. PD-1 and CTLA4 mRNA expression in CD8+ T cells treated with IgG4 was measured with real-time qPCR. The lower panel, the secretion of TNF-α, IFN-γ, and Granzyme B in CD8+ T cells treated with IgG subtypes and IVIG was measured with ELISA kits; (B) FITC-labeled IgG4 can bind to activated CD8+ T cells or NK cells, and FcR blockers can block the binding reaction. The PBS group was set as a negative control; (C) flow cytometry was used to assess the expression of CD163, CD206, ARG-1, IL-10, IL-1β, IL-12, and TNF-α in human monocytes following treatment with IgG1 or IgG4; (D) monocytes were treated with IgG1 or IgG4 (250 µg/mL) for a week. IgG1 led to M1-like macrophages, while IgG4 led to M2-like macrophages. HSA was used as the control. DAPI, 4′,6-diamidino-2-phenylindole; FcR, Fc receptor; HSA, human serum albumin;CTLA-4, Cytotoxic T lymphocyte antigen-4; IFN-γ, interferon γ; IL, interleukin; PBS, phosphate-buffered saline; TNFα, tumor necrosis factor α.

The results of monocytes tests are depicted in figure 5C,D. The presence of IgG1 induced the differentiation of monocytes into macrophages exhibiting an M1 phenotype, while the presence of IgG4 led to monocytes showing a propensity for differentiation into macrophages with an M2 phenotype. The morphological features of the differentiated cells are illustrated in figure 5D. Flow cytometry analysis confirmed that IgG4 treatment decreased M1 marker expression (IL-1β, IL-12, TNFα) and increased M2 marker expression (CD163, IL-10, CD206, ARG1), suggesting that IgG4 promotes M2 macrophage polarization in human monocytes, while IgG1 promotes M1 macrophage polarization. These findings suggest that IgG4 inhibits the growth and immune function of CD8+ T cells and monocytes in vitro by suppressing cytokine secretion and inducing inhibitory phenotype, potentially affecting immune responses.

IgG4 S228P anti-PD-1 mAb but not Fc-null IgG1 antibody promotes tumor growth in vivo

In tumor-bearing mouse models experiments, the tests were conducted in accordance with the methodology depicted in figure 6A. We observed that both nivolumab and IgG4wt displayed comparable effects in promoting tumor growth in 4T1 breast cancer and CT26 colon cancer wildtype mouse models (figure 6B,C). Tumor weight and volume in both groups were similar and significantly better than the control groups at harvest (figure 6D–G). In the humanized PD-1 4T1 mouse model, both penpulimab and nivolumab exhibited obvious efficacy. Although penpulimab displayed a tendency towards superior efficacy compared with nivolumab, no statistically significant difference was observed between the two within the observation period (figure 6B,C and H,I). Immunohistochemical staining was performed on tumor tissues from the wild-type 4T1 model using antibodies against CD3, CD4, CD8, CD163, F4/80, and CD31 (see figure 6J). The analysis of immunohistochemical staining revealed that the presence of IgG4wt and nivolumab impeded the infiltration of T cells, specifically CD8+ T cells, into the tumor, promoted the polarization of macrophages towards the M2 phenotype, and increased the level of vascularization within the tumor. In contrast, no notable differences were observed among the IgG1wt, penpulimab treated groups, and the control group (figure 6K–P). These results suggest that the S228P mutated IgG4-type PD-1 mAb maintains the immunosuppressive properties of IgG4wt.

Figure 6

IgG4 proteins promote tumor growth on mouse tumor-bearing models. (A) BALB/c mice with subcutaneous tumors were randomly assigned to groups and given different treatments, including IgG1wt, IgG4wt, penpulimab, or nivolumab. The proteins were dissolved in PBS at 2 mg/mL and given at a dose of 200 µg peritumorally every 5 days for four injections. The control group received PBS. Cancer models were created by injecting 4T1 cells or CT26 cells under the left forearm after the initial protein injection. (B–C) Tumor tissues were analyzed in groups of 4T1 breast cancer (n=5/group for wild-type, n=4/group for humanized PD-1 model) and CT26 colon cancer (n=4/group) mouse models. Tumor growth curves were plotted for each treatment group in both models. (D–I) Tumor weight results and final tumor volume in each treatment group of the three models at harvest. (J) Tumor tissues from the 4T1 model in each treatment group were analyzed using immunohistochemical staining with antibodies against CD3, CD4, CD8, CD163, F4/80, and CD31. Scale bar=50 µm. (K–O) The quantitative analysis of T cell subtype (CD3, CD4, CD8) and M2 phenotype macrophage (CD163, F4/80) infiltration was performed by randomly selecting five high power visual fields of staining tissues. (P) Tumor angiogenesis was assessed by measuring the average gray value of anti-CD31 staining. PBS, phosphate-buffered saline; PD-1, programmed cell-death 1.

Discussion