VH12 rearrangements in adult peritoneal B cells

Alternative routes to maturity: branch points and pathways for generating follicular and marginal zone B cells

Positive and negative selection of developing B cells is critical for generating a functional non-pathogenic B-cell repertoire. Newly formed B cells in the bone marrow or peripheral lymphoid system can be eliminated by one of several negative selection mechanisms or recruited through a poorly understood positive selection mechanism. In this review, we focus on the growing literature on the relevance of immature (transitional) peripheral B cells to the area of B-cell positive selection, with an emphasis on the notion that transitional B cells can be subdivided into several functionally distinct subpopulations. In this discussion, we consider the nature of these transitional B-cell subsets and their relevance to selection events that influence whether developing B cells eventually give rise to follicular versus marginal zone B cells. In addition, we attempt to initiate a resolution of current controversies surrounding transitional B-cell subsets and offer an alternative model of peripheral B-cell maturation and the follicular versus marginal zone decision.

The IgG2a/IgA produced by the murine T560 B lymphoma that arose during a graft-versus-host reaction is polyreactive and somatically mutated

In mice undergoing a graft-versus-host (GVH) reaction, donor T cells responding to the host's MHC antigens induce polyclonal activation of the host's B cells and secretion of their antibodies and autoantibodies. T560, a CD5- B lymphoma that arose in the gut-associated lymphoid tissue (GALT) of a (B10 x B10.H2aH4(b)pWts) F1 hybrid mouse that had been injected with parental B10.H2aH4b splenocytes, is of particular interest because it produces switched, heavily mutated, but, nevertheless, polyreactive immunoglobulin. T560 bears and contains IgG2a but switches to IgA spontaneously. The T560 Ig variable region is encoded by a V186.2-related VH gene, juxtaposed to DFL 16 and J(H)1, and by a Vkappa gene of the Vkappa 4/5 group juxtaposed to Jkappa1. Both VH and VK are heavily mutated. The IgA binds to polystyrene, to p-azophenyl-phosphorylcholine (PC)-conjugated keyhole limpet hemocyanin (KLH) (PC-KLH), to 2,4,6 trinitrophenylated (TNP)-KLH and to human TNF-beta but not to KLH, human TNF-alpha, or any of several other Ags tested. Hapten inhibition experiments indicate that the polystyrene, PC- and TNP-binding sites do not overlap. The switched isotypes and heavy load of somatic mutations found in the T560 IgG2a/IgA suggest that T cell-dependant somatic selection of the T560 precursor B cell may have been superimposed on polyclonal B cell activation originally associated with the GVH.

The majority of murine VH12-expressing B cells are excluded from the peripheral repertoire in adults

We have previously demonstrated that at birth most productive (P) VH12 rearrangements in B10.H-2aH-4bp/Wts (2a4b) mice encode a ten-amino acid CDR3, and that a significant fraction of the expected repertoire is absent. We have now examined the adult VH12 CDR3 repertoire involving all four JH gene segments in both peritoneum and spleen. Of the 74 P VH12 rearrangements from these tissues 67 encode a CDR3 of ten amino acids and include a Gly in the fourth position (designated 10/G4). Most of these rearrangements appear to derive from phosphatidylcholine (PtC)-specific B cells, which also have a 10/G4 VHCDR3, since few 10/G4 P rearrangements were present in spleen cells depleted of PtC-specific B cells. Thus, the VH12 B cell repertoire in adult mice is largely restricted to the use of a single CDR3 motif and to a single antigen specificity. This bias results from two selection events: (1) selective exclusion of most VH12 B cells from the peripheral repertoire, and (2) clonal expansion in the periphery of VH12 B cells that have a 10/G4 VHCDR3 and bind PtC. Analysis of VH12-JH1 rearrangements in viable motheaten (mev/mev) mice, which have an abnormal B cell repertoire due to a defective phosphatase (Hcph) and have barely detectable numbers of PtC-specific B cells, indicates that selective exclusion of VH12 B cells from the peripheral repertoire occurs normally, but that clonal expansion of 10/G4 VH12 B cells is minimal. This is evidence that the selective exclusion of VH12 B cells from the peripheral repertoire and the clonal expansion of VH12 B cells with a 10/G4 CDR3 are due to independent signaling events.

VH CDR3-dependent positive selection of murine VH12-expressing B cells in the neonate

Five to fifteen percent of peritoneal B1 (CD5+) cells from unmanipulated mice produce antibodies that bind bromelain-treated mouse red blood cells and the hapten phosphatidylcholine (PtC). The majority of these B cells express either of two VH/V kappa gene combinations, VH12/V kappa 4 or VH11/V kappa 9. Both the VH11 and VH12 genes are rearranged to JH1 and encode third complementarity determining regions (CDR3) of restricted length and sequence. These and other observations argue strongly that PtC-specific B1 cells are antigen selected. To determine when selection of PtC-specific B1 cells begins in mice we have used the polymerase chain reaction to amplify VH12-D-JH1 rearrangements from livers of fetal and neonatal mice, and determined the CDR3 encoding sequences of individual clones. We find an unusually low ratio of productive (P) to non-productive (NP) rearrangements (0.4-1.0) at both developmental stages. P rearrangements in day 1 neonates are biased in D gene use and in the sequence and length of their deduced VHCDR3. These biases are similar to those of PtC-specific B1 cells in the adult peritoneum. D gene use and CDR3 length and sequence are significantly less biased among VH12 P rearrangements 2 to 3 days earlier in the day 18 fetal liver. We suggest that this rapid change in repertoire is due to positive ligand selection that is dependent on the sequence of VHCDR3. We suggest further that the majority of VH12-expressing cells are not ligand selected and consequently undergo programmed cell death. The evidence of restriction in day 1 neonatal livers and the low P/NP ratio in the fetus suggests that selection of VH12-expressing cells begins before birth.

CD5+ B cells and the immune system

The CD5+ B-cell population is prominent in early life and may play a key role in the ontogeny of the immune system. Transplantation studies in mice are in support of CD5+ B cells as a separate lineage from CD5- B cells. In both mice and men there is evidence in favour of CD5 being an activation antigen rather than a lineage marker, but the jury is still out! The frequency of CD5+ B cells appears to be under genetic influence. CD5+ B cells are receptive to many cytokines including IL-2 and IL-5 and themselves produce a number of cytokines especially IL-10. The function of the CD5 molecule on B cells is presently unknown but it might be involved in interaction with CD72 on other B cells. CD5+ B cells generally utilise minimally mutated germ-line genes and produce low avidity auto- and polyreactive antibodies (natural antibodies) generally of the IgM class.