INTRAGRAFT EXPRESSION OF CYTOKINE TRANSCRIPTS DURING PIG PROISLET XENOGRAFT REJECTION AND TOLERANCE IN MICE

The rejection of pig proislet (islet precursor) xenografts in CBA/H mi ce is a CD4(+) T cell-dependent process. The molecular mechanisms of x enograft rejection, xenograft survival during anti-CD4 mAb therapy and xenograft tolerance post-withdrawal of anti-CD4 mAb administration, w ere examined by using a semiquantitative PCR method. Temporal analysis of intragraft cytokine mRNA demonstrated a Th0-like pattern of expres sion (IL-2, IFN-gamma, IL-3, IL-4, IL-5, and IL-10) on day 4 of the ac ute xenograft rejection process. From day 5, however, only Th2-associa ted transcripts (IL-3, IL-4, IL-5, and IL-10) were enhanced in xenogra fts compared with isograft controls. Immunohistochemistry showed that the principal participants in the rejection infiltrate were CD4(+) T c ells and eosinophils, with smaller numbers of CD8(+) T cells. In vivo depletion of CD4(+) T cells prevented xenograft rejection but had mini mal effect on the peak levels of IL-2, IFN-gamma, and IL-10 mRNA; in c ontrast, the enhanced expression of IL-3, IL-4, and IL-5 transcripts s een in rejecting xenografts was abrogated. This established a positive correlation between acute xenograft rejection, presence of CD4(+) T c ells, and enhanced intragraft expression of mRNA for the Th2-type cyto kines IL-3, IL-4, and IL-5. In tolerant hosts, long-term proislet xeno graft survival and function (>190 days) was accompanied by intragraft expression of IL-2 and IL-10 mRNA; IFN-gamma, IL-3, IL-4, and IL-5 mRN A were either undetected or not enhanced. The induced rejection of lon g-term functioning xenografts (>170 days) in nontolerant hosts resulte d in selective enhancement of IL-4 transcript expression. This study s uggests that Th2-like CD4(+) T cells are differentially activated in r esponse to xenoantigen and that xenograft tolerance is associated with lack of expression of the Th2 cytokine, IL-4.