Rheumatoid arthritis (RA) is a chronic systemic disease characterized by uncontrolled inflammation of the joints and progressive destruction of joint cartilage and bone, ultimately leading to functional impairment and disability [1]. The pathogenesis of RA is highly complicated, and its precise etiology is still unknown. As a kind of autoimmune disease, the abundance of T cells within the mononuclear infiltrates of the hyperplastic synovial membrane, taken together with the local production of T-cell-derived cytokines, suggests that T cells may be involved in the autoimmune response in RA [[2], [3], [4]]. In addition, the pathogenic roles of autoantibodies produced by B cells in RA and other autoimmune diseases have been widely studied [[5], [6], [7]].

As specialized B-cell helper cells, CD4+ follicular helper T (Tfh) cells facilitate germinal center (GC) reactions by providing signals required for B-cell differentiation and antibody production [8]. Recent evidence has indicated that overproduction of Tfh cells is responsible for autoantibody production and autoimmune disorders, such as systemic lupus erythematosus and rheumatoid arthritis [9,10]. Tfh cells highly express C-X-chemokine receptor type 5 (CXCR5), which enables homing to B-cell follicles. Moreover, these cells have an activated effector phenotype and express elevated levels of surface molecules such as CD40 ligand (CD40L), inducible T-cell costimulator (ICOS), programmed death-1 (PD-1), and B and T lymphocyte attenuator (BTLA). Mature Tfh cells induce B-cell responses through various mechanisms, including cell surface CD40L molecules and secreted soluble cytokines, such as IL-21, IL-4, and IL-9. These functional molecules synergistically promote B-cell activation, proliferation, somatic hypermutation, and differentiation into high-affinity plasma cells or memory B cells in the germinal center [11,12].

However, recent studies revealed that the contact time between Tfh cells and B cells in the germinal center is short, and a few minutes of BCR stimulation is not long enough for complete B-cell responses, B cells require multiple contacts with Tfh cells for a full differentiation and antibody production [13,14]. Whether other effector mechanisms are involved in Tfh cell-mediated B-cell differentiation still needs to be confirmed. As we know, IL-21 signal reprogramming can cooperate with CD40 and BCR signals and play a key role in B cell selection and differentiation [15,16], and Tfh cell-derived IL-9 also promotes the GC development of memory B cells [12]. In addition, a recent study also revealed that the soluble protein extracellular matrix protein 1 derived from Tfh cells is critical for GC B-cell reactions and antigen-specific antibody production [17]. Thus, we speculated that soluble components secreted by Tfh cells may also play a vital role in GC responses. Small extracellular vesicles (sEVs) are natural nanosized particles released from parental cells that participate in intracellular communication and influence recipient cell behavior via the delivery of functional biomolecules [18,19]. Notably, due to the lipid bilayer, sEVs are more stable in body fluids than cytokines and other soluble molecules. Moreover, since sEVs express characteristic surface molecules of their origin cells, they have a natural targeting ability. Thus, Tfh cells may regulate the B-cell response by releasing sEVs. Interestingly, activated T-cell-derived sEVs have been reported to induce the stimulation and proliferation of resting T cells [20]. In addition, the costimulatory molecule CD40L was found to exist in CD4+ T-cell-derived sEVs [21]. Whether Tfh cell-derived sEVs (Tfh-sEVs) can regulate B-cell differentiation and the GC response by carrying CD40L remains largely unclear.

In this study, we characterized the function of Tfh cell-derived sEVs and explored their underlying mechanisms in regulating B-cell responses, especially on B cell differentiation and antibody production. In addition, we identified the pathogenic role of Tfh-sEVs in CIA development and analyzed their clinical application value in RA patients. This study aimed to reveal a novel pattern for the regulation of Tfh cells on B cells and provide new ideas for the clinical diagnosis and treatment of RA.