CD99 signals caspase-independent T cell death.

Death signaling by Fas and TNF receptors plays a major role in the control of activated mature T cells. However, the nature of the death receptors, which may be used by the immune system to control T cells that have not acquired susceptibility to Fas ligand or TNF, is not established. In this study, we demonstrate that engagement of distinct epitopes on CD99 rapidly induces T cell death by a novel caspase-independent pathway. A new mAb to these CD99 epitopes, Ad20, induces programmed cell death of transformed T cells as determined by morphological changes, phosphatidylserine exposure on the cell surface, and uptake of propidium iodide. In general, ligation of CD99 induced kinetically faster and more profound death responses as compared with the impact of anti-Fas and TNF-related apoptosis-inducing ligand (TRAIL). Ad20-induced programmed cell death was observed with seven of eight T cell lines examined, and notably, only two of these were distinctly responsive to anti-Fas and TRAIL. CD99-mediated death signaling proceeded independently of functional CD3, CD4, CD45, and p56(lck), revealed distinctions from CD47-mediated T cell death responses, and was not influenced by interference with CD47 signaling. In contrast to the effect on transformed T cell lines, Ad20-induced death responses were not observed with normal peripheral T cells. Thus, our data suggest that CD99 is linked to a novel death pathway that may have biologic relevance in control of early T cells.

Human cytomegalovirus induces apoptosis in the hematopoietic cell line MO7e.

Several studies have shown that human cytomegalovirus (HCMV) induces growth suppression of hematopoietic progenitors. In vitro studies have demonstrated that the HCMV-induced suppression is independent of viral protein production. Previous studies have indicated a link between HCMV infection and apoptosis in human cells. The purpose of our study was to investigate whether the observed inhibitory effect of HCMV on the human myeloid progenitors could be connected to the induction of apoptosis. The growth and cell death of the hematopoietic cell line MO7e was investigated following infection with HCMV virions and dense bodies. Both virions and dense bodies inhibited the growth of MO7e cells, and induced cell death measured by trypan blue staining. In addition, both HCMV virions and dense bodies caused an increased amount of apoptosis-characteristic DNA fragmentation in the MO7e cells compared to mock-treated cells. The HCMV virions were also able to induce an increased expression of phosphatidylserine on the cell surface, which is an early event in the initiation of apoptosis in most cell types. In conclusion, HCMV and HCMV dense bodies are able to induce apoptosis in the myeloid progenitor cell line MO7e.

CD47 signals T cell death.

Activation-induced death of T cells regulates immune responses and is considered to involve apoptosis induced by ligation of Fas and TNF receptors. The role of other receptors in signaling T cell death is less clear. In this study we demonstrate that activation of specific epitopes on the Ig variable domain of CD47 rapidly induces apoptosis of T cells. A new mAb, Ad22, to this site induces apoptosis of Jurkat cells and CD3epsilon-stimulated PBMC, as determined by morphological changes, phosphatidylserine exposure on the cell surface, uptake of propidium iodide, and true counts by flow cytometry. In contrast, apoptosis was not observed following culture with anti-CD47 mAbs 2D3 or B6H12 directed to a distant or closely adjacent region, respectively. CD47-mediated cell death was independent of CD3, CD4, CD45, or p56lck involvement as demonstrated by studies with variant Jurkat cell lines deficient in these signaling pathways. However, coligation of CD3epsilon and CD47 enhanced phosphatidylserine externalization on Jurkat cells with functional CD3. Furthermore, normal T cells required preactivation to respond with CD47-induced apoptosis. CD47-mediated cell death appeared to proceed independent of Fas or TNF receptor signaling and did not involve characteristic DNA fragmentation or requirement for IL-1beta-converting enzyme-like proteases or CPP32. Taken together, our data demonstrate that under appropriate conditions, CD47 activation results in very rapid T cell death, apparently mediated by a novel apoptotic pathway. Thus, CD47 may be critically involved in controlling the fate of activated T cells.

The TCR-binding region of the HLA class I alpha2 domain signals rapid Fas-independent cell death: a direct pathway for T cell-mediated killing of target cells?

TCR binding to an MHC class I/peptide complex is a central event in CTL-mediated elimination of target cells. In this study, we demonstrate that specific activation of the TCR-binding region of the HLA-A2 class I alpha2 domain induces apoptotic cell death. mAbs to this region rapidly induced apoptosis of HLA-A2-expressing Jurkat E11 cells, as determined by morphologic changes, phosphatidylserine exposure on the cell surface, and propidium iodide uptake. In contrast, apoptosis was not induced following culture with mAbs directed to other regions of the class I molecule. Death signaling by class I molecules is apparently dependent on coreceptor activation, as apoptosis is also signaled by HLA-A2 molecules, where the intracytoplasmic residues were deleted. HLA class I alpha2-mediated cell death appeared to proceed independent of the Fas pathway. Compared with apoptotic signaling by Fas ligation, HLA class I alpha2-mediated responses displayed a faster time course and could be observed within 30 min. Furthermore, class I alpha2-induced cell death did not involve observable DNA fragmentation. The apoptotic response was not affected significantly by peptide inhibitors of IL-1beta converting enzyme (ICE)-like proteases and CPP32. Taken together, activation of the TCR-binding domain of the class I alpha2 helix may result in apoptotic signaling apparently dependent on a novel death pathway. Thus, target HLA class I molecules may directly signal apoptotic cell death following proper ligation by the TCR.