FUNCTIONAL SIGNIFICANCE OF THE 77C→G POLYMORPHISM IN THE HUMAN CD45 GENE: ENHANCED T CELL REACTIVITY BY VARIANTLY EXPRESSED CD45RA ISOFORMS

Abstract

O70 Background: A detailed knowledge of genetic variability in immunologically relevant genes is one pre-requisite to estimate the individual patient risk for rejection and/or infection. Thus, polymorphisms in several cytokine genes have been shown to influence the outcome of organ transplantation. We identified a polymorphism in the human CD45 gene (transition of C to G at position 77 in exon A; 77C→G). The mutation occurs with low frequency (2%) in healthy individuals and prevents proper splicing of exon A and thereby downregulation of CD45RA isoforms. In affected individuals CD45RA molecules are expressed on cell types which usually are CD45RA-negative (e.g. activated T cells, memory T cells). Aims: In search of functional consequences of 77C→G we investigated T cell receptor (TCR)-induced signalling events in alloreactive CD4+ T cell lines from 5 control individuals(CD45RA−CD45R0+) and 3 blood donors displaying the variant CD45RA pattern (CD45RA+CD45R0+). Results: Variant CD4+ T cells proliferated significantly stronger to stimulation with specific alloantigen than control cells. Furthermore, there was enhanced IL-2 production in variant CD4+ T cells following allogeneic stimulation. Variant and control cell lines did not differ in the relative amount of TCR-zeta, ZAP-70 and p56lck and showed a similar band pattern of tyrosine phosphorylation after antibody-mediated triggering of the TCR. However, whereas maximum intensity of tyrosine phosphorylation occurred 1 to 2 min after stimulation in control cells, the kinetics of phosphorylation was accelerated in variant T cells reaching a maximum already between 30 s and 1 min and declining after 2 min. Furthermore, the generation of p21 TCR-zeta phosphoisomers was amplified after TCR stimulation of variant cell lines compared to controls. Conclusions: These data suggest that aberrant expression of CD45RA isoforms in individuals carrying 77C→G increases the intensity of TCR signal transduction. We recently identified a group of 7 allograft recipients (1 heart, 3 kidney, 3 liver) showing the variant CD45RA expression pattern. Studies are underway to analyze whether T cell hyperreactivity may have an influence on the rejection of allografts. Supported by the Deutsche Forschungsgemeinschaft, Grant Schw437/2.