Human CD4/CD45RA+ and CD4/CD45RA− T cell subsets express CD4-p56lck complexes, CD4-associated lipid kinases, TCR/CD3-p59fyn complexes, and share similar tyrosine kinase substrates

Abstract

T cell activation appears to be regulated by an interplay between protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPases). p56^lck and p59^fyn have been found to associate with CD4 and TCR-CD3 respectively. The CD45 family of transmembrane PTPases has been shown to be able to regulate the activities of these receptor-associated PTKs in vitro. In man, CD45 contains five different isoforms whose distribution defines subsets of T cells having distinct activation requirements and in vitrofunctions. Several groups have reported a physical interaction between distinct isoforms of CD45 and CD2, CD4, and the TCR-CD3 complex. Given the potential regulatory interaction between CD45 and PTKs in CD4⁺ subsets expressing different CD45 isoforms, we have examined CD4 associated and TCR-CD3⁻ associated PTK activities, associated phosphatidyl inositoi (PI) kinases and substrates of tyrosine phosphoryiation in CD45RA⁺and CD45RA⁻ CD4⁺ T cell lines derived from peripheral blood. Both subsets express CD4-assoclated p56^lck and TCR-CD3-associated p59^fyn kinases which exhibit identical in vitro phosphoryiation at the Y-394 and Y-420 autophosphorylation sites respectively. Further, both subsets exhibited PI kinases activity associated with CD4-p56^lck. Consistent with these observations, anti-CD3 crosslinklng induced the phosphoryiation of a similar spectrum of intracellular substrates in these CD45RA⁺and CD45RA⁻ CD4⁺ T cell lines. These observations indicate that despite the possible interaction between CD45 isoforms and CD4 or TCR-CD3, the mere expression of the CD45RA isoform does not in and of itself alter the presence of receptorassociated kinases or their intracellular targets.