STAT5A/B CONTROL SURVIVAL OF T CELLS BY REGULATING THE FUNCTION OF PRO- AND ANTI-APOPTOTIC GENES DURING ALLOGRAFT REJECTION

P987 Aims: The T-cell-growth factors signaling through Janus kinases (Jak1 and 3) and signal transducer and activator of transcription (Stat1, 3, 5a/b and 6) induce full T cell response. Using the Stat5a/b double knockout, we examined the role of both proteins in acute allograft rejection. Methods/Results: Although normal Stat5a/b+/+ (H-2b) recipients acutely rejected C3H (H-2k) heart allografts (7.3 ± 0.5 days; n=4), Stat5a/b−/− mice rejected heart allografts in a delayed fashion (19.3 ± 7.6 days; n=6; p<0.01). In vitro, Stat5a/b−/− T cells failed to proliferate in response to conconavalin A (ConA) or alloantigens and at 24 and 48 hours displayed 30% and 90% apoptosis, respectively, as shown by TUNEL assay; T cells from Stat5a/b+/+ mice proliferated in response to ConA or alloantigen with only 5% apoptosis. Microarray analysis showed that pure Stat5a/b−/− T cells (but not Stat5a/b+/+ T cells) expressed multiple pro-apoptotic mRNAs (at 24 hrs after activation). Western blot confirmed that after activation Stat5a/b−/− T cells expressed elevated level of pro-apototic Bax and no anti-apoptotic Bcl-2; in contrast activated Stat5a/b+/+ T cells expressed Bcl-2 and no Bax. PCR method showed that (at 24 hrs) ConA-stimulated Stat5a/b+/+ or Stat5a/b−/− T cells produced similar amounts of IL2, IL4, IL10 and INF-γ mRNAs. Purified Stat5a/b+/+ and Stat5a/b−/− B cells proliferated in similar fashion after stimulation with LPS and showed no apoptosis. As shown by FACS, sera from both Stat5a/b+/+ and Stat5a/b−/− rejectors had identical concentrations of donor-specific IgM, IgG1, IgG2a and IgG2b alloantibodies. Conclusions: Although Stat5a/b are not required for cytokine production they are critical for regulation of apoptotic activity in T cells. In contrast, Stat5a/b deficiency does not affect B-cell function, including proliferation, differentiation and immunoglobulin class switching. Thus, following T-B cell collaboration the Stat5a/b-deficient recipients mediate allograft rejection by donor-specific IgM and IgG alloantibodies.