Fundamental roles of the innate-like repertoire of natural antibodies in immune homeostasis

Abstract

The composition of the early immune repertoire is biased with prominent expression of spontaneously arising B cell clones that produce IgM with recurrent and often autoreactive binding specificities. Amongst these naturally arising antibodies (NAbs) are IgM antibodies that specifically recognized amaged and senescent cells, often via oxidation-associated neo-determinants. These NAbs are present from birth and can be further boosted by apoptotic cell challenge. Recent studies have shown that IgM NAb to apoptotic cells can enhance phagocytic clearance, as well as suppress proinflammatory responses induced via Toll-like receptors, and block pathogenic IgG-immune complex (IC)-mediated inflammatory responses. Specific antibody effector functions appear to be involved, as these anti-inflammatory properties are dependent on IgM-mediated recruitment of the early recognition factors of complement. Clinical surveys have suggested that anti-apoptotic cell (AC) IgM NAbs may modulate disease activity in some patients with autoimmune disease. In mechanistic studies, anti-AC NAbs were shown to act in dendritic cells by inhibition of the mitogen-activated protein kinase (MAPK) pathway, a primary signal transduction pathway that controls inflammatory responses. This immunomodulatory pathway has an absolute requirement for the induction of MAPK phosphatase-1. Taken together, recent studies have elucidated the novel properties of a class of protective NAbs, which may directly blunt inflammatory responses through a primitive pathway for regulation of the innate immune system.

Keywords: immunoregulation; innate-like; natural antibody.

FIGURE 1

Model of an IgM–NAb complex that enhances interactions between an apoptotic cell and professional phagocytes. In this idealized model, apoptotic death results in membrane alterations that expose a range of neo-determinants. PC-associated membrane determinants are recognized by antigen-binding sites of a pentameric IgM. Binding of PC determinants results in conformational changes in the mu constant regions, which expose a conformational site responsible for recruitment of the globular heads of C1q (Czajkowsky and Shao, 2009). Alternatively, MBL binds to nearby high mannose N-linked glycoconjugates on mu-associated sites (not shown). This polymeric IgM–C1q complex is involved in generating or stabilizing interactions with receptors on professional phagocytes, which enhance apoptotic phagocytosis and blocks inflammatory responses.