CD19, a Response Regulator of B Lymphocytes,Regulates Wound Healing through Hyaluronan-Induced TLR4 Signaling

Immune cells are critical to the wound-healing process, through both cytokine and growth factor secretion. Although previous studies have revealed that B cells are present within wound tissue, little is known about the role of B cells in wound healing. To clarify this, we investigated cutaneous wound healing in mice either lacking or overexpressing CD19, a critical positive-response regulator of B cells. CD19 deficiency inhibited wound healing, infiltration of neutrophils and macrophages, and cytokine expression, including basic and acidic fibroblast growth factor, interleukin-6, platelet-derived growth factor, and transforming growth factor-β. By contrast, CD19 overexpression enhanced wound healing and cytokine expression. Hyaluronan (HA), an endogenous ligand for toll-like receptor (TLR)-4, stimulated B cells, which infiltrates into wounds to produce interleukin-6 and transforming growth factor-β through TLR4 in a CD19-dependent manner. CD19 expression regulated TLR4 signaling through p38 activation. HA accumulation was increased in injured skin tissue relative to normal skin, and exogenous application of HA promoted wound repair in wild-type but not CD19-deficient mice, suggesting that the beneficial effects of HA to the wound-healing process are CD19-dependent. Collectively, these results suggest that increased HA accumulation in injured skin induces cytokine production by stimulating B cells through TLR4 in a CD19-dependent manner. Thus, this study is the first to reveal a critical role of B cells and novel mechanisms in wound healing.Healing of cutaneous wounds is a complex biological event that results from the interplay of a large number of resident and infiltrating cell types, including leukocytes.¹ Accumulating evidence has revealed a critical role of leukocytes in wound healing. Infiltrating neutrophils form a first line of defense against local infections and are also a source of pro-inflammatory cytokines to activate fibroblasts and keratinocytes.² Macrophages also regulate wound healing by antimicrobial function, wound debridement, and production of various growth factors, such as platelet-derived growth factor (PDGF), transforming growth factor (TGF)-β, basic fibroblast growth factor (bFGF), heparin binding epidermal growth factor, and TGF-α.^3,4,5,6 These factors stimulate the synthesis of extracellular matrix by local fibroblasts, generate new blood vessels, promote the granulation tissue formation, and enhance re-epithelialization.^4,5 Furthermore, a series of experimental studies have indicated a significant role for T lymphocytes in wound healing as growth factor-producing cells as well as immunological effector cells.^1,7,8,9 Thus, immune cells have an integral function in wound healing beyond their role in inflammation and host defense, mainly through the secretion of signaling molecules, such as cytokines and growth factors.⁶However, little is known regarding a role of B cells in wound healing. Previous studies have revealed that B cells are present within wound tissue^9,10,11 and that B cell count is rapidly increased in the first 4 days after wounding, and thereafter reaches a plateau before falling as the wounds heal.¹¹ Furthermore, recent assessment of the role of B cells in the immune system has indicated that B cells are more than just the precursors of antibody (Ab)-secreting cells.¹² B cells have essential functions in regulating immune responses, including the production of various cytokines and growth factors, antigen presentation, regulation of T cell activation and differentiation, and regulation of lymphoid organization.¹² Therefore, the increased numbers of B cells within wound tissue may reflect a role for these cells that has hitherto been unrecognized.Both innate and adaptive immune responses contribute to host defense cooperatively. B cells play a principal role in adaptive immune response through B cell antigen receptor (BCR). BCR-induced signals are further modified by other cell surface molecules including CD19. CD19, a major positive response regulator, is a critical B cell-specific signal transduction molecule of the immunoglobulin superfamily expressed by early pre-B cells from the time of heavy chain rearrangement until plasma cell differentiation.¹³ B cells also primarily participate in innate immunity; indeed, B cells express toll-like receptors (TLRs) and respond to exogenous innate stimuli such as lipopolysaccharide (LPS), a major component of Gram-negative bacteria. CD19 also regulates LPS signaling: B cells from CD19-deficient (CD19^−/−) mice are hyporesponsive to most transmembrane signals, including BCR ligation and LPS, while B cells from CD19-transgenic (CD19Tg) mice that overexpress CD19 by ∼threefold are hyperresponsive to these signals.^14,15 Thus, CD19 regulates both adaptive and innate immune responses.In the current study, to clarify the roles of B cells in wound healing, we investigated the wound-healing model in CD19^−/− and CD19Tg mice. The results of this study indicate that CD19 controls cytokine and growth factor production by B cells mainly through TLR4 signaling, which was activated by an endogenous TLR4 ligand hyaluronan (HA) increased in the wounded skin, and thereby CD19 regulates the skin wound-healing process.