Regulatory B lymphocytes in humans: a potential role in autoimmunity.

The physiology of B cells has been revolutionized by the attribution of a number of new functions to their already established role in antibody production. Subpopulations of B cells have consequently been delineated, and Breg cells have been identified in mice, but not yet in humans. These Breg cells have the capacity to restrain immune responses and thus prevent pathogenic autoimmunity. Alteration of their function might signal an increased likelihood of disturbed immune homeostasis, making Breg cells an appealing target for immunotherapy, provided a confident characterization can be made. Recent experiments have rekindled the debate on the many alternative functions of B lymphocytes (1), which are no longer thought to have the production of antibodies as their only function. Studies initiated in genetically engineered mice and currently being extended in humans point to their role in antigen presentation (2) in the synovium of patients with rheumatoid arthritis (RA) and to their production of cytokines (3) in the blood of patients with systemic lupus erythematosus (SLE). In addition, their control of lymphoid organogenesis (4) and their function in association with dendritic cells (DCs) in ectopic germinal centers in the salivary glands of patients with primary Sjogren’s syndrome (SS) is consistent with a regulatory role of B lymphocytes. These observations revealed that autoimmune diseases could therefore all be mediated via the longterm activation of B cells (5) and that these cells may play a key role in autoimmunity. This has provided a rationale for therapeutic strategies that alter B cell survival (for review, see refs. 6 and 7). Furthermore, the diversity of their new properties has yielded the delineation of B cell populations into phenotypically unique and functionally distinct subsets. The distribution of these subsets is altered in the blood of patients with autoimmune disease (for review, see ref. 8), and the sequence of their return after therapeutic B cell depletion is abnormal (9). These studies implicate B cell subsets in the pathogenesis and/or maintenance of autoimmune diseases. Wolf and colleagues (10) have suggested a supplemental protective function of B cells against experimental autoimmune encephalomyelitis (EAE), a surrogate model of human multiple sclerosis, in B10.PL mice. The B cell–deficient B10.PL MT mice were unexpectedly unable to recover from EAE (Figure 1). Other mouse models have since confirmed that B cell depletion does not alleviate, but instead exaggerates, autoimmune traits. These findings have engendered, at least in the mouse, the concept of Breg cells, and evidence of their dominant role in immune regulation has been accumulating. Several mouse studies have allowed the dissection of mechanisms that underlie this B cell– induced regulation (for review, see ref. 11). That B cells are involved in some autoimmune conditions is not negated by such a new property. In this review, we critically describe some murine models of autoimmune rheumatic diseases and discuss some clues that suggest the existence of a human Christophe Jamin, PhD, Ahsen Morva, BSc, Sebastien Lemoine, BSc, Capucine Daridon, PhD, Pierre Youinou, MD, DSc, MACR: Brest University Medical School Hospital, Brest, France; Agnes Revol de Mendoza, PhD: Autonomous University of Nuevo Leon, Monterrey, Mexico. Dr. Youinou has received consulting fees, speaking fees, and/or honoraria (less than $10,000) from GlaxoSmithKline. Address correspondence and reprint requests to Pierre Youinou, MD, DSc, MACR, Laboratory of Immunology, Brest University Medical School Hospital, BP824, F29609 Brest, France. E-mail: youinou@univ-brest.fr. Submitted for publication December 19, 2007; accepted in revised form March 5, 2008.