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B lymphocytes are divided into two subpopulations, B1 and B2 cells based on expression of the T cell-associated protein CD5. Natural B1 cells are further divided into B1a cells that express CD5 on their membrane and B1b cells that do not but share most other biological characteristics of B1a cells. Recent studies from our laboratory have revealed, in humans, the existence of two alternative isoforms of the CD5 protein. A cell surface CD5 isoform which uses exon 1A (E1A) of the gene in B1a cells, and an intracellular isoform which uses exon 1B (E1B) mainly in human B1b cells. Indeed, the protein isoform encoded by transcripts containing E1B lack the leader peptide and is, thus, retained in the cytoplasm of B cells. The restriction of interleukin (IL)-10 to B1 lymphocytes in the mouse raises the possibility that the human CD5-E1B-expressing B cells produce IL-10. This prediction was confirmed in the CD5 negative Jok-1 B cells transfected with cDNA for either isoforms resulted in high level IL-10 production. Our data indicate that E1B-CD5-expressing B cells have the capacity to interfere with the immune response through their ability to produce high levels of IL-10.

Recent advances in molecular biological and human cell hybridization technology have significantly advanced the knowledge of mechanisms that underlie human rheumatoid factor (RF) production. These advances have provided insight into the etiopathogenesis of synovial inflammation and lymphocyte recruitment in rheumatoid arthritis (RA) joints. We have examined the mechanisms that lead to RF production in RA patients and those that regulate RF production in normals. The studies revealed structural features that distinguish RF produced in normals from those produced in RA synovial tissue. There are significant differences in the use of VL and VH genes between the two RF populations. Furthermore, IgV genes encoding synovial RF in RA have extensive evidence for nucleotide changes, leading to amino acid replacement in the complementarity determining regions (CDRs). In addition, RF produced in RA synovia show evidence for affinity maturation, isotype switch to IgG RF, and repertoire shift indicative of a continued recruitment of B cells. Together with computer modeling and crystallographic studies, our data suggest that the mechanisms that operate on RF selection in RA synovia are similar to immune responses to exogenous antigens. In contrast, RF established from human immunized donors (HID) are characterized by a very low ratio of replacement to silent (R:S) nucleotide changes in the CDR1+2. In addition, there is little increase in affinity with increasing numbers of mutations. There is thus evidence for regulatory mechanisms that limit affinity maturation of RF in normals.

Epstein-Barr virus (EBV)-immortalized monoclonal B-cell lines were established from CD5+ and CD5- cord-blood B cells. IgM from many of both CD5+ and CD5- clones reacted with IgG-Fc, ssDNA, and a variety of other autoantigens. More CD5+ B cells that used light chains of the kappa isotype reacted with IgG-Fc and ssDNA than kappa-bearing CD5- B cells. Because many of the clones reacted with IgG-Fc, they were analyzed for the expression of cross-reactive idiotypes (CRI) associated with rheumatoid factor and cold agglutinin paraproteins using murine antibodies (mAb) recognizing V kappa and VH subgroup-associated determinants. Expression of the V kappa IIIb sub-subgroup-associated idiotope recognized by 17.109 mAb was expressed at significantly higher frequency (32%; p less than 0.05) and IgM antibodies derived from the CD5+ compared with the CD5- clones (5%). Both CD5+ and CD5- clones expressed the RF paraprotein-associated idiotope recognized by G8 mAb to the same extent. Similar results were obtained using binding to SpA as a marker of VH III family usage. Furthermore, no differences in frequency of expression of RF paraprotein-associated idiotopes recognized by B6 and/or D12, and characteristic of some antibodies using VH III family genes, were found between the CD5+ and CD5- populations. Although a higher than expected frequency of VH IV-gene expression was demonstrated (around 30%) in both CD5+ and CD5- cells, there were differences in expression of CRI recognized by mAb Lc1 and R2.1A2 with specificities for two VH IV subfamilies. While some CD5+ and CD5- clones were identified in which their IgM reacted with mAb Lc1, only CD5+ clones were recognized by another mAb R2.1A2. Analysis of the relationships between antigen specificities and V kappa- and VH-family gene usage indicated that auto- or polyreactivity was not associated with V kappa III nor any particular VH family. The higher frequency of the V kappa IIIb sub-subgroup-associated idiotope recognized by 17-109 in the CD5+ clones and the association of CD5+ B cells with the VH IV subfamily recognized by mAb R2.1A2 and 9G4 may suggest that CD5+ B cells in cord blood are expanded as a result of recruitment within the fetal environment.