In this study we have demonstrated that leptin activates B cells from aged humans and induces significantly higher amounts of TNF-α, IL-6, and IL-10 as compared to B cells from young subjects. This is in part due to an increased phosphorylation of STAT3.
Aging is considered a state of chronic inflammation. Plasma levels of proinflammatory cytokines, including IL-6 and TNF-α are increased in human aging [6–11]; however, the cellular source of increased proinflammatory cytokines is poorly understood. These cytokines are produced by T cells, macrophages, and B cells. However, macrophage-derived IL-1β, TNF-α and IL-6 and Th1 derived cytokines are decreased in aging [16–20]. Therefore, B cells is likely one of the sources of increased proinflammatory cytokines associated with human aging, though dendritic cells in aging may also contribute to increased levels of proinflammatory cytokines . Recently, B cells have shown to produce a number of cytokines, chemokines, and growth factors with proinflammatory properties [21–25].
Leptin is one of adipokines, which is powerful inducer of inflammatory cytokines by Th1 T cells and macrophages, including IFN-γ, IL-1β, TNF-α, and IL-6 [1–5, 27, 28]. More recently, we have reported that leptin also activates human B cells to induce inflammatory cytokines, including IL-6 and TNF-α, and anti-inflammatory/ immunoregulatory cytokine IL-10 [29–31]. We have reported that leptin activates human B cells , and serum levels of leptin are increased in aging [17–20]. Since leptin and proinflammatory cytokines are increased in aging, and T cell-and macrophage-derived proinflammatory cytokines are decreased in aging, we investigated the effects of leptin on purified B cells. In the present study we show that leptin activates human aged B cells to a greater extent than B cells from young subjects as evidenced by upregulation of CD25 and CD69, which was observed on both CD19 + CD5+ B cells and CD19 + CD5-B cells, greater upregulation was observed in CD5+ B cells as compared to CD5- B cells. CD5+ B cells are reported to produce IL-10 with immunoregulatory property , which may explain a role of leptin in autoimmunity [1, 33].
Activation of B cells was also associated with increased production of cytokines by B cells. Leptin induced significantly higher (P < 0.05) secretion of IL-6, TNF-α, and IL-10 by B cells (ELISA assay) from aged humans as compared to B cells from young subjects. Furthermore, we demonstrate that leptin not only induced increased amounts of secreted cytokines by aged B cells, but leptin also increased significantly higher number B cells (ELISPOT assay) to secrete these cytokines. Increase in leptin-induced IL-10 in aging may suggest a compensatory mechanism to counterbalance increased IL-6 and TNF-α production. Since B regulatory cells produce IL-10  it is possible that increased IL-10 production might be due to increased Breg in aging. This possibility is under investigation. IL-10 rescues CD5+ cells from apoptosis and augments the proliferation of CD5- B cells [35–39]. Since IL-6 and TNF-α are produced by both CD5+ and CD5- B cells, increased leptin-induced B cell-derived IL-10 in aging may be playing a role in aging-associated chronic inflammation by surviving B cells from apoptosis to produce IL-6 and TNF-α. Buffa et al.  have reported an expansion of CD19 + CD38-CD24- B cells in aged humans and these cells produce both TNF-α and IL-10.
Leptin signal via three distinct pathways, including JAK-STAT, PI3K, and MAPK pathways [41–43]. Following binding of leptin with leptin receptor, JAK2 (Janus kinase 2) is activated by auto-or cross phosphorylation. Activated JAK2 tyrosine phosphorylates intracellular domain of leptin receptor. The phosphorylated receptor provides a docking site for STAT (signal transducer and activator of transcription) factors, particularly STAT3, which is substrate of JAK2. STAT3 itself is a substrate for JAK2 and homo- or hetero-dimerization upon phosphorylation, translocates to the nucleus, and modulates transcription of genes, including cytokine genes. We show that Leptin induced significantly greater phosphorylation of STAT3 in B cells from aged subjects as compared to B cells from young controls, suggesting that leptin-induced increased cytokine production by aged B cells is via increased activation of STAT3.