Functional significance of the 77C --> G polymorphism in the human CD45 gene: enhanced T-cell reactivity by variantly expressed CD45RA isoforms

Outcome of children treated with haematopoietic-stem cell transplantations from donors expressing the rare C77G variant of the PTPRC (CD45) gene

The uncommon C77G polymorphism of the Protein-Tyrosine Phosphatase (PTPRC) gene (PTPRC; previously termed CD45) could confer an increased risk of immunopathology. This study compared the outcome of children following human leucocyte antigen-matched unrelated haematopoïetic-stem cell transplantations (HSCT) from donors carrying (C77G cases: n = 8) or not (controls: n = 36) the PTPRC C77G polymorphism. Transmission of the PTPRC C77G polymorphism through the graft was suggested by unusual CD45RA phenotype in the donors and/or in the recipients after, but not before HSCT. Restriction-Fragment Length Polymorphism and sequencing confirmed the polymorphism. Overall survival rates were similar in C77G cases and controls (63% vs. 61%). Acute leukaemia relapse tended to be less frequent in C77G cases (0% vs. 32%; P = 0·09). Among recipients surviving ≥ 30 d, acute GVHD (aGVHD) ≥ grade 2 tended to be more frequent (100% vs. 58%; P = 0·07) and the rate of steroid-refractory or -dependant aGVHD higher (67% vs. 28%) in C77G cases. Finally, extensive chronic GVHD tended to occur more frequently (40% vs. 9%) in C77G cases. Recovery of lymphocyte subsets and virus-specific CD4 was similar in C77G cases and controls while interleukin 2 (IL2)-responses through CD3 stimulation were higher in C77G cases (P = 0·004). In conclusion, HSCT from PTPRC C77G donors could increase GVHD risk without compromising overall survival. Altered IL2-responses could be involved in this process.

Altered CD45 isoform expression in C77G carriers influences cytokine responsiveness and adhesion properties of T cells

The C77G polymorphism in exon A of the human CD45 gene occurs with low frequency in healthy individuals. An enhanced frequency of C77G individuals has been reported in cohorts of patients suffering from multiple sclerosis, systemic sclerosis, autoimmune hepatitis, hepatitis C and human immunodeficiency virus (HIV)-1. C77G individuals overexpress CD45RA isoforms on activated/memory T cells. We have shown previously that aberrant expression of CD45RA isoforms enhances the intensity of T cell receptor (TCR) signalling. Here we report that the C77G polymorphism also influences the responsiveness of T cells to cytokines and alters their adhesion properties. When stimulated by interleukin (IL)-2, C77G T cells proliferated more strongly than wild-type controls and showed accelerated phosphorylation of Janus kinase (Jak1). Furthermore, C77G T cells exhibited a higher tendency to form homotypic aggregates in culture which could be enhanced significantly by antibody-mediated triggering of the variant CD45RA molecules. These data indicate that the changes in CD45 isoform combination resulting from C77G may not only affect TCR signalling but also cytokine-driven T cell responses and cellular adhesion. Altered immune responsiveness may enhance susceptibility of C77G carriers for certain diseases.

Unusual case presentations associated with the CD45 C77G polymorphism

CD45, the leucocyte common antigen, is a haematopoietic cell specific tyrosine phosphatase. Human polymorphic CD45 variants are associated with autoimmune and infectious diseases and alter the phenotype and function of lymphocytes, establishing CD45 as an important regulator of immune function. Here we report four patients with diverse diseases with unusual clinical features. All four have the C77G polymorphism of CD45 exon 4, which alters the splicing and CD45RA/CD45R0 phenotype of lymphocytes. We suggest that C77G may be a contributing factor in these unusual cases.

CD45 negatively regulates tumour necrosis factor and interleukin-6 production in dendritic cells

CD45 is known to regulate signalling through many different surface receptors in diverse haemopoietic cell types. Here we report for the first time that CD45-/- bone marrow dendritic cells (BMDC) are more activated than CD45+/+ cells and that tumour necrosis factor (TNF) and interleukin-6 (IL-6) production by BMDC and splenic dendritic cells (sDC), is increased following stimulation via Toll-like receptor (TLR)3 and TLR9. Nuclear factor-kappaB activation, an important downstream consequence of TLR3 and TLR9 signalling, is also increased in CD45-/- BMDC. BMDC of CD45-/- mice also produce more TNF and IL-6 following stimulation with the cytokines TNF and interferon-alpha. These results show that TLR signalling is increased in CD45-/- dendritic cells and imply that CD45 is a negative regulator of TLR and cytokine receptor signalling in dendritic cells.

CD45 regulates apoptosis in peripheral T lymphocytes

Programmed cell death (apoptosis) is a key mechanism for regulating lymphocyte numbers. Murine lymph node lymphocytes cultured in vitro without added stimuli show significant levels of apoptosis over 24 h, detectable by staining with Annexin V. CD4 and CD8 T lymphocytes from transgenic (Tg) mice expressing single CD45RABC or CD45RO isoforms show increased apoptosis and the extent of apoptosis is inversely correlated with the level of CD45 expression. CD45 Tg cells exhibit phosphatidyl serine translocation and DNA oligonucleosome formation, and can be partially rescued from apoptosis by culture in caspase inhibitors or common gamma-chain-binding cytokines. We conclude that CD45 is an important regulator of spontaneous apoptosis in T lymphocytes and this mechanism may contribute to the disease associations reported for individuals expressing CD45 variant alleles.

Geographical distribution and disease associations of the CD45 exon 6 138G variant

CD45 is crucial for normal lymphocyte signalling, and altered CD45 expression has major effects on immune function. Both mice and humans lacking CD45 expression are severely immunodeficient, and single-nucleotide polymorphisms in the CD45 gene that cause altered splicing have been associated with autoimmune and infectious diseases. Recently, we identified an exon 6 A138G polymorphism resulting in an increased proportion of activated CD45RO T cells and altered immune function. Here we report a significantly reduced frequency of the 138G allele in hepatitis C Japanese patients and a possibly reduced frequency in type I diabetes. The allele is widely distributed in the Far East and India, indicating that it may have a significant effect on disease burden in a large part of the human population.

The 77C->G mutation in the human CD45 (PTPRC) gene leads to increased intensity of TCR signaling in T cell lines from healthy individuals and patients with multiple sclerosis

The 77C-->G mutation in exon A of the human CD45 gene occurs with low frequency in healthy individuals. An enhanced frequency of 77C-->G individuals has been reported in cohorts of patients suffering from multiple sclerosis, systemic sclerosis, autoimmune hepatitis, and HIV-1. To investigate the mechanisms by which the variant allele may contribute to disease susceptibility, we compared T cell reactivity in heterozygous carriers of the mutation (healthy individuals and multiple sclerosis patients) and wild-type controls. In vitro-generated T cell lines and freshly isolated CD4+CD45R0+ primed/memory T cells from 77C-->G individuals aberrantly expressed CD45RA isoforms and showed enhanced proliferation and IL-2 production when stimulated with anti-TCR/CD3 mAb or Ag. Mutant T cell lines contained a more active pool of p56lck tyrosine kinase and responded with increased phosphorylation of Zap70 and TCR-zeta and an enhanced Ca2+ flux to TCR/CD3 stimulation. These data suggest that 77C-->G may act as a risk factor for certain diseases by increasing the intensity of TCR signaling.

Altered CD45 expression and disease

CD45, the leucocyte common antigen, is a haemopoietic cell-specific tyrosine phosphatase. Many isoforms are generated by alternative splicing, but their function remains obscure. The extracellular domain of CD45 is highly polymorphic in all vertebrates. Importantly, human polymorphic variants that alter CD45 isoform expression are associated with autoimmune and infectious diseases, establishing CD45 as an important immunomodulator with a significant influence on disease burden. Here, we discuss the new opportunities provided by the human variants for investigating and understanding how CD45 regulates antigen receptor signalling, cytokine responses and apoptosis.

Altered CD45 expression in C77G carriers influences immune function and outcome of hepatitis C infection

BACKGROUND

A polymorphism in exon 4 (C77G) of CD45 that alters CD45 splicing has been associated with autoimmune and infectious diseases in humans.

OBJECTIVE

To investigate the effect of C77G in hepatitis C virus (HCV) infected individuals and study the phenotype and function of peripheral blood mononuclear cells (PBMC) from healthy and hepatitis C infected C77G carriers.

RESULTS

C77G individuals showed an increased proportion of primed CD45RA and effector memory CD8 T cells and more rapid activation of the lymphocyte specific protein tyrosine kinase (Lck) following CD3 stimulation. Transgenic mice with CD45 expression mimicking that in human C77G variants had more activated/memory T cells, more rapid proliferative responses, and activation of Lck.

CONCLUSIONS

Changes in CD45 isoform expression can alter immune function in human C77G variants and CD45 transgenic mice. The C77G allele may influence the outcome of HCV infection.