MHC Disparate Resting B Cells Are Tolerogenic in the Absence of Alloantigen-Expressing Dendritic Cells

Resting B cell (rB) populations have been shown to tolerize to soluble proteins and to minor-H but not to MHC alloantigens. We speculated that the reason for failing to tolerize to MHC alloantigen is that the few remaining dendritic cells (DCs) contaminating purified rB cell populations efficiently activate MHC allospecific T cells which are present at a higher frequency than T cells specific for minor-H alloantigen and soluble proteins. We established that MHC disparate rB cells are indeed tolerogenic when devoid of DC populations, as parental strain mice showed delayed skin graft rejection when infused with rB cells from mice in which MHC class I alloantigen was specifically targeted to T and B cells (CD2- transgenic mice). In contrast, treatment of parental strain mice with allogeneic rB cells purified from MHC- transgenic mice, in which is ubiquitously expressed, including DCs, induced accelerated graft rejection. We also showed that adding only 5,000 expressing DCs to CD2- rB cells abrogated the tolerogenic effect. Surprisingly, allogeneic rB cells prolonged graft survival in -primed mice. Thus, MHC disparate rB cells are tolerogenic and their failure to delay graft rejection can be explained by contaminating allogeneic DCs. 1. Introduction Resting B cells have been suggested to be potent inducers of specific unresponsiveness both in vitro and in vivo [1, 2] and have been thought to mediate such effects by directly presenting Ag (signal 1) in the absence of productive costimulatory interactions, principally mediated by B7-CD28 and CD40-CD40L interactions (signal 2) [3–6]. This hypothesis, however, has been challenged by studies demonstrating that rB cells not only express CD40, but also upregulate expression of B7 family members in a timely fashion in vivo [7]. Moreover, it has become increasingly appreciated that indirect antigen presentation, in which antigen shed from donor cells is presented via immature host DC, may be crucial for tolerance induction [8–11]. Regardless of the mechanism, rB cells clearly have tolerogenic effects on T cells specific for some types of antigen, but not others. For instance, rB cells induce tolerance to soluble proteins [2, 12] and to minor-H mismatched transplants [1], but not to transplants discrepant in MHC alloantigens [7]. That MHC disparate grafts present a higher bar for tolerance induction in general is emphasized by studies showing that MHC-expressing allografts are rejected even in the absence of the “danger” signals that appear requisite in generation of immune responses to other antigens, presumably by