Costimulatory function and expression of CD40 ligand, CD80, and CD86 in vascularized murine cardiac allograft rejection

Recent data implicates a role for the CD40- CD40 ligand (CD40L) pathway in graft rejection. One poten- tial mechanism is direct costimulation of T cells through CD40L. Alternatively, the ability of CD40 stimulation to induce CD80 (B7-1) and CD86 (B7-2) expression on antigen- presentingcells(APCs)hasledtothehypothesisthattherole ofCD40-CD40Linteractionsintransplantrejectionmightbe indirect, i.e., to promote the costimulatory capacity of APCs. Here, we have used a murine vascularized cardiac allograft modeltotestthishypothesis.Treatmentoftherecipientswith donor splenocytes and a single dose of anti-CD40L mAb induces long-term graft survival (>100 days) in all animals. This is associated with marked inhibition of intragraft Th1 cytokine (interferon g and interleukin (IL) 2) and IL-12 expression with reciprocal up-regulation of Th2 cytokines (IL-4 and IL-10). In untreated allograft recipients, CD86 is stronglyexpressedonendothelialcellsandinfiltrating mono- nuclear cells of the graft within 24 hr. In contrast, CD80 expression is not seen until 72 hr after engraftment. Anti- CD40L mAb has no detectable effect on CD86 up-regulation, but almost completely abolishes induction of CD80. However, animals treated with anti-CD80 mAb or with a mutated form of CTLA4Ig (which does not bind to CD86) rejected their cardiac allografts, indicating that blockade of CD80 alone does not mediate the graft-prolonging effects of anti-CD40L mAb. These data support the notion that the role of CD40- CD40L in transplant rejection is not solely to promote CD80 or CD86 expression, but rather that this pathway can directly andindependentlycostimulateTcells.Thesedataalsosuggest that long-term graft survival can be achieved without block- ade of either T cell receptor-mediated signals or CD28-CD86 engagement.