Clinical characteristics of pSS patients testing negative for anti-SSA and anti-SSB antibodies

A total of 934 patients with pSS were included in this study (Fig. 1), among whom 800 (85.7%) were women. All included patients satisfied the 2002 AECG classification criteria and/or the 2016 ACR/ EULAR classification criteria of pSS. None of included pSS patients had other CTD. The median age of included patients was 58 (49–66) years, and the median disease duration was 36 (10–96) months (Table 1).

Table 1 Clinical characteristics of pSS patients with and without anti-SSA and anti-SSB antibody negativity

Overall, 299 (32.0%) patients demonstrated negativity for both anti-SSA and anti-SSB antibodies. A total of 635 (68.0%) patients demonstrated positivity for one or two of the antibodies; among them, 630 and 254 tested positive for anti-SSA and anti-SSB antibodies, respectively. Among patients with anti-SSA or anti-SSB antibody positivity, 218 underwent MSGB; 194 (97.7%) demonstrated positive pathological findings. All of the 299 patients who tested negative for anti-SSA and anti-SSB antibodies had positive MSGB results. The other autoantibodies in the 934 patients were distributed as follows: 258 (27.6%) had an ANA titer of ≥ 1:320 and 533 (57.1%), 69 (7.4%), 66 (7.1%), 81 (8.7%), and 380/864 (44.0%) had anti-Ro52, anti-CENP-B, anti-RNP, AMA-M2, and RF positivity, respectively. The distribution of autoantibody positivity in the cohort is shown in Fig. 2.

Fig. 2

Distribution of autoantibodies in patients with primary Sjögren’s syndrome. Data are expressed as n (%). ANA, antinuclear antibodies; anti-CENP-B, anti-centromere protein B; anti-RNP, anti-ribonucleoprotein; AMA-M2, anti-mitochondrial M2 antibody; RF, rheumatoid factor

As shown in Table 1, pSS patients with anti-SSA and anti-SSB antibody negativity were older (61.7 ± 10.2 vs. 54.8 ± 13.1 years, p < 0.001), had shorter disease duration (24 [6–60] vs. 48 [12–108] months, p < 0.001), and had a lower proportion of females (75.3% vs. 90.6%, p < 0.001) than those with anti-SSA or anti-SSB positivity. Compared with patients having anti-SSA or anti-SSB positivity, those testing negative for these antibodies had a lower prevalence of palpable purpura (4.1% vs. 7.6%, p = 0.045). However, compared with pSS patients having anti-SSA or anti-SSB positivity, those testing negative for these antibodies had a higher prevalence of abnormal Schirmer I tests (96.0% vs. 89.1%, p = 0.001), and ILD (59.2% vs. 28.8%, p < 0.001). No statistical differences were found between the two groups in terms of nationality, dry mouth, dry eyes, fatigue, arthralgia, dental caries, parotid enlargement, lymphadenopathy, and focus scores.

Hematological characteristics of pSS patients testing negative for anti-SSA and anti-SSB antibodies

As shown in Table 2, patients testing negative for anti-SSA and anti-SSB antibodies demonstrated lower ANA positivity than those testing positive for anti-SSA or anti-SSB antibodies (18.7% vs. 31.8%, p < 0.001), anti-Ro52 antibodies (23.7% vs. 72.8%, p < 0.001), and RF (22.2% vs. 54.4%, p < 0.001); however, they demonstrated higher positivity for anti-CENP-B (12.7% vs. 4.9%, p < 0.001). Compared with patients testing positive for anti-SSA or anti-SSB antibodies, patients testing negative for these antibodies demonstrated a lower prevalence of leucopenia (13.4% vs. 32.4%, p < 0.001), neutropenia (3.7% vs. 10.2%, p = 0.001), anemia (10.7% vs. 23.5%, p < 0.001), and thrombocytopenia (6.7% vs. 13.6%, p = 0.002); the findings were similar for hyper-IgA (20.9% vs. 29.8%, p = 0.004), hyper-IgG (28.3% vs. 53.5%, p < 0.001), low C3 (15.9% vs. 24.1%, p = 0.005), low C4 (25.4% vs. 38.2%, p < 0.001), and elevated ESR (39.5% vs. 54.4%, p < 0.001).

Table 2 Imunologic and hematologic characteristics of pSS patients with and without anti-SSA and anti-SSB antibody negativity

As shown in Fig. 3, compared with patients having anti-SSA or anti-SSB antibody positivity, those testing negative had higher counts of leucocytes (5.76 [4.58, 7.05] vs. 4.76 [3.71, 6.31], p < 0.001), neutrophils (3.42 [2.56, 4.46] vs. 2.74 [1.97, 4.02], p < 0.001), lymphocytes (1.69 ± 0.66 vs. 1.50 ± 0.61, p < 0.001), and platelets (207 [161, 251] vs. 186 [145, 234], p = 0.001) and higher hemoglobin levels (129 [120, 139] vs. 122 [110, 132], p < 0.001); the findings for C3 (0.89 [0.78, 0.99] vs. 0.82 [0.71, 0.96], p < 0.001) and C4 (0.18 [0.14, 0.22] vs. 0.20 [0.16, 0.24], p < 0.001) levels were similar. Compared with patients testing positive for anti-SSA or anti-SSB antibodies, those testing negative had lower levels of IgA (2.68 [1.83, 3.57] vs. 2.95 [2.16, 3.98], p = 0.001) and IgG (13.5 [11.15, 17.00] vs. 16.90 [13.13, 21.40], p < 0.001). No statistical differences were found in terms of IgM levels.

Fig. 3

Imunologic and hematologic distribution of patients with and without anti-SSA and anti-SSB antibody negativity. The uppermost and lowermost lines represent the maximum and minimum values of the data, respectively; the upper-line and lower-line of the boxplot represent the third and first quartiles, respectively; the thick line segment in the middle of the box plot represents the median of the data. IgG, immunoglobulin G; IgA, immunoglobulin A; IgM, immunoglobulin M; C3, complement 3; C4, complement 4

Factors associated with anti-SSA and anti-SSB antibody negativity in patients with pSS

In multivariate regression analysis, the following variables were included: age, sex, disease duration, nationality, arthralgia, palpable purpura, Schirmer I test results, ANA titers, anti-Ro52 positivity, anti-CENP-B positivity, RF positivity, leucopenia, neutropenia, anemia, thrombocytopenia, levels of IgA, IgG, C3, and C4, ESR, and ILD. No multicollinearity was found between the independent variables after the covariance diagnosis.

As shown in Table 3, anti-SSA and anti-SSB antibody negativity in patients with pSS was positively associated with age (OR = 1.03, 95% CI: 1.01, 1.05), male sex (OR = 1.86, 95% CI: 1.05, 3.31), ILD (OR = 2.54, 95% CI: 1.67, 3.85), anti-CENP-B positivity (OR = 4.17, 95% CI: 2.04, 8.53), and abnormal Schirmer I test results (OR = 2.85, 95% CI: 1.24, 6.53). Anti-SSA and anti-SSB antibody negativity was negatively associated with disease duration (OR = 1.00, 95% CI: 0.99, 1.00), anti-Ro52 positivity (OR = 0.13, 95% CI: 0.09, 0.19), RF positivity (OR = 0.40, 95% CI: 0.25, 0.63), and thrombocytopenia (OR = 0.47, 95% CI: 0.24, 0.96).

Table 3 Factors associated with anti-SSA and anti-SSB antibody negativity in patients with pSS.