ppt+notes

SLIDE 1:

CD95 and CD95L are transmembrane proteins of the tumor necrosis factor family. CD95 also belongs to the subfamily of death receptors characterized by an intracellular 'death domain'. Binding of CD95L to the receptor results in a signal cascade leading to the biochemical and morphological changes characteristic of apoptosis.CD95 is expressed in many tissues, whereas expression of the apoptosis-inducing ligand CD95L is restricted to a few cell types and is regulated on the transcriptional level by various transcription factors. There are multiple types of CD95L, the human soluble form is highly cytotoxic and the murine soluble form blocks CD95L-mediated apoptosis by competing with the membrane-bound form for CD95 binding.Interaction with antigen first leads to clonal expansion of T cells, followed by a down-phase of the immune response several days later, in which most antigen-specific T cells are eliminated in a process called activation-induced cell death (AICD). T-cells are resistant to CD95-mediated apoptosis during the expansion phase, in the down-phase they become CD95-sensitive. CD95L may also be involved in the maintenance of immune privilege. In immune privileged sites, such as eye and testis, deleterious immune reactions are prevented by reduced MHC expression, secretion of immunosuppressive cytokines and physical barriers.

SLIDE 2:

Contrary to typical conditions in the body, where only T-cells present CD95L, tumor cells of diverse origin have been reported to express CD95L. These tumors include hepatocellular carcinoma, lymphoma, melanoma, astrocytoma, esophageal carcinoma, gastric adenocarcinoma and others. Tumor cells with stable CD95L expression become apoptosis-resistant because they up-regulate CD95L while at the same time down regulate CD95, resulting in tumor counterattack against T-cells and resistance to both self CD95L interaction and T-cell interaction. However, T-cells are not always vulnerable to CD95-mediated apoptosis. Short-term activated T cells are resistant to CD95-mediated apoptosis; however, activated T cells in culture for more than 4 days became sensitive to apoptosis induction by CD95L expressing tumor cells. In addition, tumors present CD95L expressing dendritic cells which down regulate T-cell proliferation in the human body.

SLIDE 3:
====(C) Tumor progression C57BU6 (WT), C57BU6-Ipr, and C57BU6-gld mice. The number of mice (in %) with palpable tumors are indicated. Data are representative of three to five experiments with 9 to 13 mice per group. Tumor size was measured daily. (D) Tumor progression in MRL and MRL-/pr mice. Data are representative of three experiments with 9 to 12 mice per group. ====

(C)-tumor cells mutated to express lots of CD95L
====(D)-allogenic tumor cells mutated to express lots of CD95L (should elicit more of an immune response because mouse immune system should see the allogenic tumor cells as foreign) ====

====Possible reason CD95L expression by tumors causes inflammatory response because tumor cells apoptose surrounding cells. This causes macrophages to "eat" the dead cells and then possible to recognize the tumor cell mass itself. ====

==== However, expression of CD95L on tumor cells or grafts does not confer immune privilege in vivo, but, instead, induces inflammation and accelerates rejection. Hence, presently it is not entirely clear whether tumor counterattack really is a relevant immune escape mechanism of tumors [70, 76 ]. ====

==== Since neurons and astrocytes also express CD95L [17, 25 , 78 ], immune privilege in the central nervous system may also involve CD95L. However, this has been contested since lpr and gld mice did not show an exacerbated experimental allergic encephalomyelitis (EAE), a model of multiple sclerosis, but instead, showed ameliorated clinical signs ==== of EAE [79 ]. The development of a Th1 response or inflammatory cell infiltration into the central nervous system was not affected.

Interestingly, although CD95L+ tumor cells could kill activated T cells, CD95L expression of the tumor had no significant influence on the cytotoxicity of T cells against tumor cells in vitro (Fig. 2 ). T cells that were stimulated with alloantigen or tumor antigen lysed CD95L+ and CD95L tumor cells in vitro to the same extent [39, 56 ]. Alloantigen stimulated human T cells lysed CD95L+ and CD95L tumor cells in a 4-h assay in a similar fashion [22 ]. In general, in cytotoxicity assays

A number of reports contradicting the tumor counterattack hypothesis have also been published. In contrast to the above findings, it has been reported that CD95L expression by grafts or by tumor cells led to rejection, neutrophil infiltration and induction of immunity.

==== In addition, CD95Lexpressing hearts from transgenic mice transplanted into sygneneic and allogeneic recipients were more rapidly rejected than control grafts and showed neutrophil infiltration as early as one day after transplantation. ====

Possible reasons for controversial results:

====<span style="font-family: Arial,Helvetica,sans-serif;">-First of all, most of these studies are being done either in mice or in vitro. Scientist exploring these pathways can not hope to duplicate all of the complexities found in the human body, so it is understandable that results may vary. Mice CD95L should not be expected to react exactly the same to human CD95L. ====

====<span style="font-family: Arial,Helvetica,sans-serif;"> -Second, a high amount of T-cells used during in vitro studies may affect the results. Typically only 4% of tumor infiltrating cells are cytotoxic T cells, but in vitro tests skew the ratio towards much higher quantities of T-cells. Hence, the outcome of the T cell-tumor cell interaction in vivo may not directly be inferred from these in vitro findings. ====

====<span style="font-family: Arial,Helvetica,sans-serif;">-Additionally, researchers tested the killing strength (aka cytotoxic capabilities) of the tumor cells with CD95L. They found that even though the tumors expressed CD95L, they weren't able to kill infiltrating T-cells because they had reduced cytotoxicity. ====

====<span style="font-family: Arial,Helvetica,sans-serif;">Chemotherapies: Better understanding of the CD95 pathway may lead to more effective chemotherapies. One treatment would 'enhance' T-cells giving them immunity to the CD95L pathway. This would allow T-cells to interact with tumors without suffering from tumor counterattack. The second treatment would eliminate tumor's abilities to present CD95L on the surface, once again allowing T-cells to eliminate the tumor. The challenge here would be to localize the removal of CD95L to tumor cells only, as the elimination of CD95L from T-cells would render the cells impotent. ====

====<span style="font-family: Arial,Helvetica,sans-serif;">Tissue grafts: In tissue grafts, allogeneic immune responses are initiated by dendritic cells from both host and donor origins. The dendritic cells of both origins present major histocompatibility complex (MHC) which results in an allogenic immune response called graft vs host disease, or GVHD. Essentially, T-cells recognize foreign cells and launch T-cells to destroy the non-self cells. To prevent GVHD, scientist developed a buffer of hybrid DC cells called XS106 DCs or killer DCs. The killer DCs were developed with the ability to induce apoptosis in T-cells through the CD95 pathway. On the figure above we see an experiment run with the implementation of the buffer. All mice, 3 million non-self cells were grafted onto the spleen. In half, the buffer was implemented. The results appear promising with the mice receiving the killer DC buffer living an additional 12 days. ====

<span style="font-family: Arial,Helvetica,sans-serif;">Autoimmune: Some autoimmune diseases, the most notable being autoimmune lymphoproliferative syndrome (ALPS), work through the CD95 pathway to cause harm to the body. There is extreme difficulty in treating these diseases because of the complexity of the immune system, but with further understanding of CD95, hopefully advancements can be made in the treatment of diseases like ALPS.