The strategies to approach autoimmune disorders displaying elevated Ig (IgM) levels in the absence of obvious immunization are yet unclear. These disorders include hyper-IgM syndromes and other pathological conditions such as systemic lupus erythematosus (SLE). In some cases, therapeutic intervention with the administration of i.v. Igs was shown to have beneficial effects (16
), but it appears paradoxical that in steady-state conditions in which Ig levels are already increased and induce pathology, further increases in circulating Ig could still have any beneficial effects. Above and beyond the possible anti-inflammatory effects of the administered Igs (23
), the present findings showing that there are feedback mechanisms that regulate the number of activated B cells and plasma IgM levels and that these mechanisms involve plasma IgG- and FcγRIIB-mediated, SHIP1-dependent negative signals, may provide another explanation to this phenomenon.
It has been long known that passively administered Ag-specific IgG can suppress or enhance specific primary thymus-dependent (24
) and thymus-independent IgM Ab responses (26
). It is generally accepted that IgG-mediated suppression is Ag specific (24
). The suppressive effects could either be due to epitope masking and Ag sequestration by the high-affinity IgG preventing initiation of new IgM responses (25
), or be mediated by an Fcγ-dependent regulation of B cell activation [for review, see (24
)]. Trials on whether F(ab’)2
fragments kept the suppressive activity of intact IgG have given contradictory results (27
). It has also been reported that IgG suppressed specific Ab responses as efficiently in FcγR-deficient mice as in wild-type controls (28
), questioning the role of this inhibitory receptor in the control of immune responses. However, FcγRIIB-deficient mice downregulate late responses to immune complexes (30
) or during inflammation (31
), suggesting that these receptors may act to prevent overextended immune responses (27
). It should be pointed out that studies that investigated the in vivo role of IgG-mediated regulation of B cell activation and immune responses have all used actively immunized mice, under conditions in which high titers of Ag-specific Abs were produced (24
). The novelty of our approach resides on the fact that we performed our studies in the absence of an Ag-specific immune response. In these novel conditions, our results show that circulating IgG, constitutively present in nonimmunized mice, may have a fundamental role in controlling natural activated B cell numbers and the IgM they produce. They seal a hiatus and provide new valuable information as they demonstrate that FcγRIIB-dependent negative regulation controls B cell activation without any intentional immunization.
We show that regulation of IgM production engages a FcγRIIB-mediated, SHIP1-dependent inhibitory feedback loop in which B cells themselves sense the number of activated B cells by detecting their secreted IgG. When there are enough activated B cells, the high titers of IgG and self-reactive Abs that they produce can lead either to FcγRIIB-mediated aggregation by immune complexes, possibly inducing activated B cell apoptosis (33
), or to coaggregation of BCR and FcγRIIB, which dampens B cell activation (21
). Either mechanism may prevent the accumulation of activated IgM-secreting B cells. However, whereas direct FcγRIIB cross-linking may inhibit B cells independently of their antigenic specificity and affect a large fraction of the cells, BCR and FcγRIIB coaggregation will preferentially inhibit self-reactive B cells. Considering the stronger effects observed using SHIP1-deficient B cells, it is possible that alternative FcγRIIB-independent inhibitory pathways may also participate in the control of the IgM-secreting cell pool. SHIP was indeed shown to control activation/proliferation signals triggered by a variety of receptors that activate PI3K (13
). Once IgG levels are re-established normal B cell activation can proceed in response to new antigenic stimulation.
Our current findings might provide an evolutionary explanation for the expression of inhibitory Fcγ receptors by B cells. By simultaneously allowing detection of plasma IgG levels and inhibiting B cell activation, these receptors contribute to control the number of all Ig-secreting B cells (IgM and IgG), and thus may prevent autoimmune disorders. They indicate that self-reactive B cells can expand and secrete autoreactive Igs if not held in check by sufficient IgG in their environment. This notion is supported in part by our observation that self-reactive IgM titers produced by a B cell population were selectively reduced in the presence of a first B cell population () and by other previously published observations. The inability of B cells to detect plasma IgG because of defects in FcγRIIB expression or signaling leads to increased Ig levels and autoimmune disease (12
). Altered human FcγRIIB signaling in B cells resulting from the I232
T polymorphism (38
) as well as decreased FcγRIIB expression in memory B cells (40
) have been reported in SLE patients. Conversely, partial restoration of FcγRIIB expression in the B cells of FcγRIIB−/−
) or FcγRIIB overexpression in B cells reduces SLE incidence (42
), findings that indicate the B cell phenotype is likely to be due to FcγRIIB deficiency rather than to other genetic variation (43
). Finally, the failure to produce IgG either because of early thymectomy in toads (44
), inappropriate T–B cell cooperation in humans (5
) (), or the intrinsic inability of activation-induced cytidine deaminase–mutant B cells to switch to IgG production in both mice and humans (4
) results in hyper-IgM syndromes and autoimmune pathology. They may also be of relevance to the perception of the proposed hygiene hypothesis (45
). Indeed, increase incidence of infections in the developing countries may lead to higher serum titers of IgG, and thus help to prevent development of autoimmune and allergic diseases.
Our findings may also suggest an as yet unsuspected physiological role for natural self-reactive Abs. Specifically, it has never been clear why natural self-reactive Abs should be allowed to exist; but our observation of IgG-mediated feedback regulation in normal nonimmunized mice suggests that natural self-reactive Abs, which make up a significant fraction of the natural plasma Ig (46
) by facilitating FcγRIIB aggregation, provide a warning signal that reflects B cell density. Thus, natural self-reactive Abs might participate in activated B cell homeostasis under physiological steady-state conditions.
In conclusion, these results identify a new mechanism of homeostatic control. To date, in mammals, homeostatic control was believed to be due to competition of cellular populations for a common niche of restricted size, defined by the ensemble of cellular interactions and trophic factors required for cell survival (8
). However, in situations where resources are not limiting, that is, immune responses, excess of self-Ags or cytokines, it is not clear which mechanisms could limit expanding lymphocyte numbers. Thus, the question is as follows: how do lymphocyte populations count the number of their individuals and how do they know when to stop growing? We now show that, in absence of competing cell populations, passively administered IgG reduced activation and IgM production by subsequently introduced B cells. Thus, we propose that control of lymphocyte numbers could also be achieved by the ability of lymphocytes to perceive the density of their own populations (47
). Such mechanism would be reminiscent of the primordial quorum-sensing systems used by some bacteria in which a bacterium senses the accumulation of bacterial-signaling metabolites secreted (by the same or other cells), allowing the bacterium to sense the number of cells present in a population and adapt their growth accordingly (14
). In summary, these quorum-sensing mechanisms allow bacteria to coordinate their gene expression according to the density of their population (14
). Quorum sensing can play a critical role in lymphocyte homeostasis with the proviso that lymphocytes have the capacity to assess the number of molecules they interact with and can mount a standard response once a threshold number of molecules is detected. The situation described in this work seems to support this hypothesis. We found that homeostasis of the innate IgM-secreting B cell pool is achieved when total B cell populations are able to monitor the number of activated B cells by detecting products secreted by some of their members (). The ability of B cells to detect plasma IgG levels through such a quorum-sensing mechanism may be crucial to immune system homeostasis, providing a critical checkpoint mechanism that may prevent excessive B cell activation and autoimmunity. We speculate that malfunction of this quorum-sensing mechanism may lead to uncontrolled B cell activation and autoimmune disease (). It is possible that similar quorum-sensing mechanisms may work to maintain the homeostasis of other lymphocyte populations (47
) or control organ size during development.