CDB25:0003289 CD72 — CD5

ABSTRACT: The CD5 molecule is expressed on T cells and, at a lower density, on a minor B cell subset (CD5+ B cells). The pan-B Ag CD72 was recently identified as the CD5 counterstructure, and several data suggest the involvement of this ligand pair in T-B cell cognate interaction. However, the functional role of CD5 and CD72 molecules within the B cell compartment is still unknown. In this work we studied umbilical cord blood CD5+ B cells (B-1a), adult splenic CD5- B cells (B-2), and CD5+ B cells from patients with chronic lymphocytic leukemia. Flow cytometry analysis and proliferation assays were used to determine 1) the ability of B-1a and B-2 cells to coexpress functionally relevant counterligands other than CD5 and CD72, and 2) the signaling capacity of CD5 and CD72 in terms of B cell activation and proliferation. To this purpose, freshly isolated or preactivated normal and neoplastic B cells were cultured with agonistic anti-CD5 or anti-CD72 mAbs in the presence or the absence of cytokines equipped with B cell activity. Our data demonstrate that CD5 and CD72 molecules are coexpressed with other ligand pairs usually involved in T-B cell cognate interaction on B-1a cells, but not on B-2 cells. CD5 and/or CD72 engagement delivers critical costimulatory signals in B-1a, B-2, and B cells from patients with chronic lymphocytic leukemia, but with different requirements and patterns. Besides suggesting the potential involvement of B-1a lymphocytes in B-B cell interactions during T-independent B cell responses, our results indicate that CD5 and CD72 counterstructures play a functional role in the B cell compartment.

ABSTRACT: Recently, we reported that the CD5 protein can bind to the B cell antigen CD72. So far, no functional evidence has been given for this interaction. We used soluble native CD5 and two anti-CD72 monoclonal antibodies, JT3 and WL225, produced and characterized in our laboratory in order to investigate the role of CD5 in B cell activation. Neither the CD5 nor the antibodies induced thymidine incorporation when added to resting human B cells, but they produced a two- to five-fold increase in thymidine uptake of B cells activated using immobilized anti-sIgM mAb when compared to the cultures stimulated by anti-sIgM mAb alone. The CD5 protein was effective at concentrations as low as 0.15 microM and its effect could be abolished by preincubation with soluble recombinant CD72 but not by preincubation with control proteins, indicating the specificity of the binding. The two antibodies but not the soluble CD5 produced a costimulatory effect when B cells were stimulated with suboptimal concentrations of anti-CD40 mAb or IL-4. Altogether these data suggest that a costimulatory signal can be delivered to human B cells by CD5/CD72 interaction. The possible role of CD5/CD72 signalling in physiologic humoral responses is discussed here.