Helper T lymphocyte unresponsiveness to cardiac allografts following transient depletion of CD4-positive cells. Implications for cellular and humoral responses.

Initial treatment of heterotopic cardiac transplant recipients with anti-CD4 mAb promotes long-term (> 60 days) allograft survival. We have used modified limiting dilution analysis to quantitate donor alloantigen-reactive helper T lymphocytes (HTL) and CTL in mice bearing long-term cardiac allografts. Despite repopulation of lymphoid tissues with CD4+ T cells, donor alloantigen-reactive IL-2 producing and IL-4-producing HTL were rare or not detectable in lymphoid tissues or in the graft. While donor-reactive precursor CTL were present in lymphoid tissues, modified limiting dilution analysis revealed that stimulated ("antigen-conditioned") CTL were not detectable, and few CTL were present in the graft. In addition, antibodies reactive with donor alloantigens were not detectable in the sera of mice bearing long-term cardiac allografts. To determine whether additional in vivo stimulation with donor alloantigens would elicit an immune response, sponge allografts were implanted into mice bearing long-term cardiac allografts. Previous reports from this laboratory have demonstrated that T cell infiltration of sponge allografts is dependent upon antigen-driven cytokine production. While third-party sponge allografts were readily infiltrated by third-party-reactive HTL and CTL, sponge allografts of the same strain as the cardiac allograft were not infiltrated by T cells. However, donor strain sponge allografts induced an IgM (but not IgG) alloantibody response. These data indicate that IgM alloantibody could be induced in the absence of a cellular response to the sponge allograft. Kinetic studies revealed that a transient IgM (but not IgG) response was induced by the initial cardiac transplantation in the absence of CD4+ cells. These IgM alloantibodies disappeared by day 21 despite the persistence of the allograft. These observations indicate that transient depletion of CD4+ T cells induces allograft-specific T cell tolerance, but does not eliminate the ability to mount an allograft-specific IgM response. To our knowledge, this is the first report of a transient humoral response to alloantigens that occurs in the absence of CD4+ T cells, and can be reinduced in "tolerant" animals that fail to mount a cellular immune response. Potential mechanisms involved in the development and maintenance of anti-CD4 mAb-induced tolerance are discussed.