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Mammalian cells are capable of committing "active suicide" or apoptosis in response to specialized pathological mechanisms employing a phylogenetically developed intrinsic program of death, triggered by signal transduction through specific receptors. Changes in cellular structure such as: 1) condensation of the nuclear (chromatin) and cytoplasmic structures (especially the mitochondria); 2) blebbing of the cell membrane; 3) characteristic swelling of the endoplasmic reticulum; and 4) fragmentation of the cells in membrane bound apoptotic bodies, are the dramatic signs of total cell destruction. Apoptosis requires energy in the from of ATP, indicating that programmed cell death (PCD), as opposed to necrosis, is an energy dependent, active physiological and pathophysiological phenomenon. During this immunocytochemical study, we observed the presence of PCD in the prenatal thymus and various human neoplastically transformed tissues. During the intrauterine ontogenesis, in thymocytes or resting T lymphocytes, p53 tumor suppressor protein was identified to be a critical mediator of PCD in response to DNA damage. The cellular interaction of immature, cortical thymocytes (characterized by a double positive CD4+CD8+TCRlow immunophenotype-IP) with thymic RE cells induces positive selection of T lymphocytes that recognize, but are not activated, by self-MHC molecules (tolerance induction). Double positive CD4+CD8+CD3- thymocytes undergo FasL-mediated apoptosis, while CD4+CD8+CD3+ cells use the CD3 mediated pathway of PCD. Two step, apoptotic cell death is mainly restricted to the CD4+CD8+TCR dull thymocyte subpopulation. T-lymphocytes which do not undergo positive selection are killed by apoptosis in response to a number of intrinsic and extrinsic factors, such as chemical toxins, viral infections, X- and UV irradiation, mild hyperthermia, the actions of various hormones, extracellular survival factors, calcium ionophores (such as A23187), various chemotherapeutic drugs (adriamycin, actinomycin D, etc) and antibodies directed to the CD3-TCR (T cell receptor) complex. Immature thymocytes also undergo a second selective process, so-called negative selection, when thymic stromal cells eliminate autoreactive T lymphocytes. As a typical model of embryonal neoplasms, we observed 34 childhood PNET/MED tissues samples. A systematic observation for the presence of apoptosis related markers (especially FasR) and cells in PCD was carried out. A strong expression (intensity of staining: "A"--the highest possible; number of stained neoplastic cells: +++ to ++++, between 50% to 90%) of FasR was detected. We also observed 42 childhood glial tumors, divided as follows: 6 pilocytic ASTRs; 14 low grade ASTRs; 16 anaplastic ASTRs; and 6 GBMs. The GBMs represent an end-stage brain tumor IP dedifferentiation of glial origin. During the immunocytochemical screening of these 42 childhood ASTRs, we detected strong expression (intensity of staining: "A"--the highest possible; number of stained cells: ++ to ++++, between 20% to 90%) of FasR, employing 4 microns thick, formalin fixed, paraffin-wax embedded tissue slides. FasR expression was rated high, 70% to 90% on the tumor cells in pylocytic ASTRs, lowered to 50% to 60% on the neoplastic cells in low grade ASTRs, even lower between 30% to 40% in anaplastic ASTRs and significantly lower, between 20% to 35% on the neoplastically transformed cells of GBM tissues. The presence of apoptotic neoplastic cells was also regularly detected in other human adult neoplasms, such as thyroid, pancreatic, hepatocellular, gastric, colon, breast, ovarian, prostata, and renal cell carcinomas, as well as, in Hodgkin and non-Hodgkin lymphomas and some sarcomas. The expression of apoptosis related cell surface molecules on the surface of both neoplastically transformed cells and on tumor cell specific, cytotoxic T lymphocyte (CTL) surfaces (FasR-FasL system) raises a distinct possibility of active PCD induction in CTL by tumor cells. Juxtacrine interactions between CTL and neoplastically transformed cells, coupled with observations that tumor cells can modulate the intracellular, signaling domains of cell surface receptors to elicit responses quite often contrary to the expected, may even provide a way for CTL to enhance the proliferation and dedifferentiation of cancer cells. Adoptive cellular immunotherapies employing CTL raised against autologous neoplastically transformed cells in vitro should be employed in the control of minimal residual disease following surgical resection of the primary malignant growth.