Viral FLIP Impairs Survival of Activated T Cells and Generation of CD8+ T Cell Memory

Abstract

Viral FLIPs (vFLIPs) interfere with apoptosis signaling by death-domain-containing receptors in the TNFR superfamily (death receptors). In this study, we show that T cell-specific transgenic expression of MC159-vFLIP from the human Molluscum contagiosum virus blocks CD95-induced apoptosis in thymocytes and peripheral T cells, but also impairs postactivation survival of in vitro activated primary T cells despite normal early activation parameters. MC159 vFLIP impairs T cell development to a lesser extent than does Fas-associated death domain protein deficiency or another viral FLIP, E8. In the periphery, vFLIP expression leads to a specific deficit of functional memory CD8+ T cells. After immunization with a protein Ag, Ag-specific CD8+ T cells initially proliferate, but quickly disappear and fail to produce Ag-specific memory CD8+ T cells. Viral FLIP transgenic mice exhibit impaired CD8+ T cell responses to lymphocytic choriomeningitis virus and Trypanosoma cruzi infections, and a specific defect in CD8+ T cell recall responses to influenza virus was seen. These results suggest that vFLIP expression in T cells blocks signals necessary for the sustained survival of CD8+ T cells and the generation of CD8+ T cell memory. Through this mechanism, vFLIP proteins expressed by T cell tropic viruses may impair the CD8+ T cell immune responses directed against them.

Transgenic overexpression of the Caspase‐8 inhibitor FLIPshort leads to impaired T cell proliferation and an increased memory T cell pool after staphylococcal enterotoxin B injection

“…Interestingly, two recent reports using transgenic mouse lines with T cell-specific overexpression of cellular c-FLIP L and viral MC159-vFLIP describe suppressed in vitro AICD (although in the case of c-FLIP L AICD in vivo was unaffected [30,42]). T cell death upon TCR restimulation in vitro is driven mostly by CD95 [39], and it is presently unclear why neither FLIP S in our transgenic T cell model nor FLIP L in another transgenic study [23] protect against in vitro AICD, despite the observed resistance against cell death induced by recombinant CD95L.…”

“…Obviously, the absence of CD95 receptor signaling and blockage of caspase-mediated apoptosis induced by CD95 activation are not functionally equivalent. The difference may be explained by a recently discovered caspase-independent cell death pathway emanating from the CD95 receptor, which involves the death domain-containing kinase RIP [41].Interestingly, two recent reports using transgenic mouse lines with T cell-specific overexpression of cellular c-FLIP L and viral MC159-vFLIP describe suppressed in vitro AICD (although in the case of c-FLIP L AICD in vivo was unaffected [30,42]). T cell death upon TCR restimulation in vitro is driven mostly by CD95 [39], and it is presently unclear why neither FLIP S in our transgenic T cell model nor FLIP L in another transgenic study [23] protect against in vitro AICD, despite the observed resistance against cell death induced by recombinant CD95L.…”

“…In a recently published transgenic study, the viral MC159-vFLIP protein was expressed under the control of the CD2 enhancer in mouse T cells. Only one mouse line expressing MC159-vFLIP could be generated, and in these animals, thymocyte numbers were reduced during the first 3 months of age [42]. After the peak of thymopoeisis, the difference disappeared.…”

Inhibition of caspase-8 activity promotes protective Th1- and Th2-mediated immunity toLeishmania majorinfection

“…Transgenic mice expressing vFLIP in T cells (CD2-vFLIP) were obtained from the U.S. National Institutes of Health (Bethesda, MD, USA) [31], housed and bred at the Taconic (Germantown, NY, USA) and UFRJ transgenic animal facilities. All animal experiments were approved by the Ethics Committee for Use of Animals of UFRJ, in accordance with national regulations.…”

“…It is noteworthy, however, that caspase-8 activity is also required for other aspects of T cell activation and development of immune responses to viral and parasite infections [13, 31, 34 -37]. As we found previously that vFLIP expression negatively affects the production of cytokines by CD8 T cells and CD8 T cell-mediated immunity [31,35], we chose the L. major infection model to explore the effects of caspase-8 inhibition on CD4 T cells. Despite expressing increased Th2 cytokines along with a robust Th1 response, vFLIP mice exerted a better control over L. major infection, with reduced lesions and parasite loads, than WT mice.…”