Tolerance to cardiac allografts via local and systemic mechanisms after adenovirus-mediated CTLA4Ig expression

Blockade of the CD28/B7 T cell costimulatory pathway prolongs allograft sur vival and induces tolerance in some animal models. We analyzed the efficacy of a CTLA4Ig-expressing adenovirus in preventing cardiac allorejection in rats, the mechanisms underlying heart transplant acceptance, and whether th e effects of CTLA4Ig were restricted to the graft microenvironment or were systemic. CTLA4Ig gene transfer into the myocardium allowed indefinite graf t survival (>100 days vs 9 +/- 1 days for controls) in 90% of cases, wherea s CTLA4Ig protein injected systemically only prolonged cardiac allograft su rvival (by up to 22 days). CTLA4Ig could be detected in the graft and in th e serum for at least 1 year after gene transfer. CTLA4Ig gene transfer indu ced local intragraft immunomodulation at day 5 after transplantation, as sh own by decreased expression of the IL-2R and MHC II Ags; decreased levels o f mRNA encoding for IFN-gamma, inducible NO synthase, and TGF-beta; and inh ibited proliferative responses of graft-infiltrating cells, Systemic immune responses were also down-modulated, as shown by the suppression of Ab prod uction against donor alloantigens and cognate Ags, up to at least 120 days after gene transfer. Alloantigenic and mitogenic proliferative responses of graft-infiltrating cells and total splenocytes were inhibited and were not reversed by IL-2. In contrast, lymph node cells and T cells purified from splenocytes showed normal proliferation. Recipients of long term grafts tre ated with adenovirus coding for CTLA4Ig showed organ and donor-specific tol erance. These data show that expression of CTLA4Ig was high and long lastin g after adenovirus-mediated gene transfer. This expression resulted in down -modulation of responses against cognate Ags, efficient suppression of loca l and systemic allograft immune responses, and ultimate induction of donor- specific tolerance.