M2 Macrophages Serve as Critical Executor of Innate Immunity in Chronic Allograft Rejection

Abstract

Allograft functional failure due to acute or chronic rejection has long been a major concern in the area of solid organ transplantation for decades. As critical component of innate immune system, the macrophages are unlikely to be exclusive for driving acute or chronic sterile inflammation against allografts. Traditionally, macrophages are classified into two types, M1 and M2 like macrophages, based on their functions. M1 macrophages are involved in acute rejection for triggering sterile inflammation thus lead to tissue damage and poor allograft survival, while M2 macrophages represent contradictory features, playing pivotal roles in both anti-inflammation and development of graft fibrosis and resulting in chronic rejection. Macrophages also contribute to allograft vasculopathy, but the phenotypes remain to be identified. Moreover, increasing evidences are challenging traditional identification and classification of macrophage in various diseases. Better understanding the role of macrophage in chronic rejection is fundamental to developing innovative strategies for preventing late graft loss. In this review, we will update the recent progress in our understanding of diversity of macrophage-dominated innate immune response, and reveal the roles of M2 macrophages in chronic allograft rejection as well.

Keywords: ECM; IRI; chronic allograft rejection; fibrosis; inflammation; macrophage; solid organ transplantation; vasculopathy.

Figure 1

Polarization and distinct functions of macrophages. Macrophages derive from monocytes in the circulation and polarize into M1/M2-like phenotypes. M1-like macrophages promote tissue injury by producing pro-inflammatory cytokines including TNF-α, IL-6, IL-1β, and IL-12, while M2 represent tissue repair and fibrosis by expressing anti-inflammatory and fibrogenic mediators such as IL-10, IL-13, and TGF-β1 in stressed liver.

Figure 2

M2-like macrophages are unlikely to be exclusive for fibrotic rejection. TGF-β1 produced by M2-like macrophages facilitates differentiation of fibroblasts into myofibroblasts that extensively produce ECM, promoting chronic fibrotic allograft rejection. During this process, Th1 cells may inhibit the activation of TGF-β by producing IFN-γ, while Th2 cells represent opposite feature via secretion of IL-4/IL-13.