Bone Marrow Cell Rejection by Innate Immune Pathways Is Exclusively Dependent On the TRIF Pathway.

Abstract 3526

Poster Board III-463

Research on transplantation rejection, tolerance, and immunosuppressive agents has been directed toward the adaptive immune response as, until now, it has been believed to be the dominant mechanism for alloreactivity. The role of innate immunity in transplantation has not been fully defined. Despite the sentinel role of innate immunity in driving and shaping adaptive immunity, the contribution of innate immunity to bone narrow cells has only recently been identified. TLR4-driven MyD88-dependent immunity has been demonstrated to be critical for allogeneic innate immune responses to solid organ transplants. Whether innate immunity plays a role in BMC rejection has not been defined. In the present studies we tested whether the absence of MyD88 would enhance allogeneic engraftment in bone marrow transplantation (BMT). The core component of TLR signaling is activation of an IL-1-like pathway dependent upon the adapter MyD88. MyD88^−/− B6 mice (n=6) were used to study the role of MyD88 signaling in allogeneic BMC rejection. As TRIF is another crucial adapter for TLR3 signaling and responsible for induction of signaling via type 1 IFNs, B6 TRIF^−/− mice (n=6) were also investigated. Wild-type B6 (H2^b, n=6) mice were used as controls. In the present study, MHC plus minor antigen-disparate BALB/c (H2^d) mice served as allogeneic bone marrow donors. Recipients were nonmyeloablatively conditioned with anti-CD154 mAb (day0 and +3) to block CD40:CD154 co-stimulatory pathway and sirolimus (day0 to +4) to block late-stage T cell activation by inhibiting IL-2 responsiveness. Recipients were transplanted with 15 × 10⁶ allogeneic (BALB/c; H2^d) marrow cells 4-6 hours following conditioning with 100 cGy total body irradiation (TBI). Allogeneic engraftment was achieved in 33.3% of MyD88^−/− recipients at 1 mo after BMT, a percentage similar to that for wild-type B6 mice. Surprisingly, 100% of TRIF^−/− mice engrafted. The level of donor chimerism in TRIF^−/− mice was 5.1 ± 0.6% at one month after BMT, which was significantly higher than in MyD88^−/− (1.8 ± 1.0%; P < 0.0001) or in wild-type B6 mice (1.3 ± 0.8%; P < 0.005) and thereafter through all time points tested (2, 4 and 5 months, P < 0.05). The levels of donor chimerism increased with time and the level of donor chimerism reached 14.7 ± 7.1% TRIF^−/− at five months. These results suggest that allogeneic BMC rejection is uniquely independent of the MyD88 pathway but rather involves TRIF signaling. These data suggest that the innate immune response to BMC differs from that for solid organs and may reflect the specificity of BMT compared with other transplants. These findings demonstrate for the first time that BMC elicit an innate immune response via the TRIF pathway and may open the door to novel approaches for immune-based conditioning to promote engraftment.