Leukocyte common antigens (LCAs, also known as T200 and CD 45) are integral membrane proteins expressed exclusively on hematopoietic cells. These molecules exhibit varying molecular masses and epitopes when expressed in different cell types. To determine the genetic bases for the generation of this diversity, three classes of human LCA cDNA clones that are different near their 5' ends have been isolated. These differences arose as a result of differential usage of three exons as determined from an analysis of a genomic DNA clone. Furthermore, Northern blot analysis with LCA exon-specific probes demonstrates the existence of at least two more LCA mRNA forms that are generated by differential splicing. A comparison of the human and mouse LCA protein sequences revealed a marked difference only in the extracellular domain.

SummaryWe previously showed that fibronectin (FN) synergized with anti-CD3 in induction of CD4+ T cell proliferation, and that VLA-5 acted as a functional FN receptor in a serum-free culture system . In the present study, we showed that VLA-4 is also involved in this CD3-dependent CD4 cell activation through its interaction with the alternatively spliced CS1 domain of FN . When highly purified CD4 cells were cultured on plates coated with anti-CD3 plus synthetic CS1 peptide-IgG conjugate, significant proliferation could be observed . Neither CS1 alone nor anti-CD3 alone induced this activation. This proliferation was completely blocked by antiVLA01 (4134) and anti-VLA-4 (8F2), while antiVLA 5 (monoclonal antibody [mAb] 16 and 2H6) had no effect . These data indicate that VLA-4 mediates CD3-dependent CD4 cell proliferation via the CS1 domain of FN . AntiVLA-4 also partially (10-40%) inhibited CD4 cell proliferation induced by native FN plus anti-CD3, implying that the CS1 domain is active in the native plasma FN . However, this native FN-dependent proliferation was entirely abolished by addition of anti-VLA-5 alone. Moreover, when native FN-coated plates were pretreated with anti-FN (mAb 333), which blocks RGDS sites but not CS1 sites, no CD4 cell activation could be observed. These results strongly suggest that CD4 cell activation induced by plasma FN/anti-CD3 may be dependent on both VLA4/CS1 and VLA5/RGDS interactions, although the latter interaction may be required for function of the former.

SummaryThe VLA/integrins are a family of heterodimeric adhesion receptors shown to be involved in cell-to-cell and cdl-to-extracellular matrix (ECM) interactions. Given recent evidence that VLA molecules can synergize with the CD3/T cell receptor (TCR) pathway to activate T calls, it is important to identify biochemical event(s) generated by these molecules. Here, we report that the engagement of VLA-4 on T cells with specific antibodies or its ligand activates proteintyrosine kinase (FIX) activity as detected by antiphosphotyrosine immunoblotting. The crosslinking of VLA-/31 (CD29) with a specific monoclonal antibody (mAb) (anti-4B4) plus anti-mouse immunoglobulin resulted in the rapid tyrosine phosphorylation of a 105-kD protein (ppl05) in the human T cell line H9, as well as in peripheral resting T cells. The increase in tyrosine phosphorylation of ppl05 was specifically mediated by VLA-4, since mAbs against or4, but not against other VLA c~ chains, could induce this phosphorylation. In addition, the binding of T cells with the CS1 alternatively spliced segment of fibronectin (the binding site recognized by VLA-4) induced ppl05 tyrosine phosphorylation. Crosslinking the CD3 complex or VLA-4 molecules with mAbs demonstrated that each of these molecules stimulated the tyrosine phosphorylation of unique sets of proteins with different kinetics, suggesting that these two receptor systems are coupled to distinct PTKs. Since tyrosine phosphorylation of cellular proteins has been shown to be a crucial biochemical event in cell growth, our findings suggest that the induction of ppl05 tyrosine phosphorylation via VLA-4 may play a role in the transduction of activation signals through this molecule.