Chemokines and B-cell Homing to Follicles

Abstract

B cells that bind autoantigen in the periphery may be excluded from lymphoid follicles and rapidly eliminated (Cyster, 1997). To understand the basis for follicular exclusion we considered whether Gi coupled chemokine receptors might play a role by testing the effect of treatment with pertussis toxin (PTX), an inhibitor of Gi signaling, on B cell migration into splenic follicles. Strikingly, PTX treated B cells were unable to migrate into follicles or the white pulp cords of the spleen, whereas cells treated with buffer alone or with the oligomer B subunit of PTX could migrate into follicles normally (Cyster and Goodnow 1995). These observations led us to consider which chemokine receptors and chemokines might have a role in B cell positioning within lymphoid organs. We focused on two orphan receptors, BLR1 and EBI1, because these had been shown to be constitutively expressed by B cells in humans (Birkenbach et al. 1993; Dobner et al. 1992). To track expression of the mouse receptors, the amino-terminal ectodomains were expressed as GST fusion proteins and used to immunize rabbits. An antiserum against BLR1 was isolated and affinity purified using the same BLR1 fragment expressed as a fusion protein with mannose-binding protein. Flow cytometric analysis of mouse lymphoid tissues showed BLR1 expression on all mature B cells (Schmidt et al. 1998) with slightly higher surface expression on B cells with a CD21^hiIgD^lo marginal zone phenotype (Fig. 1). BLR1 expression was also observed on B220⁺CD5⁺ peritoneal B-l cells (Fig. 1). In B cell development, there was little or no BLR1 detectable on B220⁺IgM^- pro/pre-B cells, whereas B220⁺IgM⁺ immature B cells showed weak expression (Fig. 1; note that as BLR1 is detected with a polyclonal antiserum it is necessary to be cautious in interpreting the significance of weak signals such as seen on many of the cells in bone marrow gate G4). BLR1 expression became strongly upregulated on immature B cells at about the same time as surface IgD and CD21 (Fig. 1). The low expression by immature B cells is consistent with findings that immature B cells are inefficient at entering follicles (Cyster 1997) and suggests that BLR1 upregulation may be an important part of the immature to mature B cell transition.