Cytotoxic T cells infiltrating a glioma express an aberrant phenotype that is associated with decreased function and apoptosis
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In this study, we report on novel alterations found in rat intracranial (i.c.) tumor-infiltrating T lymphocytes (TIL) that are indicative of T cell defects and death. FACS analysis showed that the cytotoxic T cells (CTL) infiltrating rat T9.F gliomas were CD3ɛ+, αβTCR+, CD8α+, but CD8β−. These lymphocytes also stained positive for the B cell-specific marker, CD45RA, as well as Annexin-V, signifying apoptotic changes. Functional and biochemical analyses were performed to assess whether the aberrant phenotype was linked to other defects. When CD8α+ TIL were purified and stimulated in vitro, their proliferative capacity was markedly diminished in comparison with CD3+CD8α+CD8β+ T cells isolated from the spleens of naive, non tumor-bearing rats. Furthermore, the mean fluorescence intensity of surface CD3ɛ was dramatically reduced in the CD3+CD8α+CD8β− TIL population as compared with CD3+CD8α+CD8β+ TIL from the same tumor-bearing animal. Biochemical studies revealed that the expression of TCRζ and LAT were reduced in lysates generated from CD8α-purified TIL with respect to CD8α-purified T cells from naive spleen. We believe that these degenerative changes are reflective of chronic T cell receptor ligation, because in vitro culture of rat splenocytes or purified T cells with ConA or anti-CD3 mAb induced the same alterations. In vitro, the downregulation of CD8β could be inhibited by the caspase inhibitor, z-VAD. These results suggest that the aberrant CTL phenotype found in the TIL of glioma-bearing rats may be novel signals for their impending death and degenerating anti-tumor immune function.
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