The effect of CD28/B7 blockade on alloreactive T and B cells after liver cell transplantation

Background. Hepatocyte cell lines are beginning to be developed as universal donors for isolated liver cell transplantation, which is a less invasive method than orthotopic liver transplantation for treatment of metabolic liver disease. The immune response to isolated liver cell transplantation and its modification by costimulatory blockade are as yet not well delineated. Methods. Adenovirus expressing CTLA4Ig was used to study blockade of the costimulatory CD28/B7 pathway in murine models of hepatocyte transplantation, and the effects on alloreactive T and B cells were studied. Results. CTLA4Ig delayed rejection of subcutaneously administered C57L-derived murine hepatoma cells in CBA/J recipients for >50 days. Activation and cytokine secretion by allospecific CD4⁺ and CD8⁺ T cells were initially blocked by CTLA4Ig; delayed rejection was associated with tumor infiltration by CD8⁺ T cells that did not secrete interferon-γ. CTLA4Ig failed to block transplant rejection in primed mice, indicating that memory effector T cells were resistant to its action. In contrast, CTLA4Ig suppressed both naive and memory alloreactive B cells. High levels of CTLA4Ig mediated acceptance of hepatoma cells delivered directly into the spleen. However, isolated primary C57BL/6 mouse hepatocytes delivered into the spleen were rejected with only moderately delayed kinetics. Conclusions. Transplant antigenicity, transplant site, and CTLA4Ig dose all affected the survival of transplanted liver cells. CD8⁺ T cells are significant mediators of hepatocyte transplant rejection and are relatively resistant to costimulatory blockade with CTLA4Ig. Strategies to specifically antagonize CD8⁺ T cells or to modulate MHC class I expression in association with costimulatory blockade by CTLA4Ig may enhance the clinical feasibility of trans planting allogeneic hepatocytes.