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Down-regulation of the Th2-like response induced by ovalbumin-alum (OVA/alum) immunization by heat-killedBrucella abortus was not reversed by anti-IL-12 antibody treatment or in gamma interferon (IFN-γ) knockout mice, suggesting that induction of Th1 cytokines was not the only mechanism involved in theB. abortus -mediated inhibition of the Th2 response to OVA/alum. The focus of this study was to determine whether an alternative pathway involves alteration in expression of costimulatory molecules. First we show that the Th2-like response to OVA/alum is dependent on B7.2 interaction with ligand since it can be abrogated by anti-B7.2 treatment. Expression of costimulatory molecules was then studied in mice immunized with OVA/alum in the absence or presence ofB. abortus . B7.2, but not B7.1, was up-regulated on mouse non-T and T cells following immunization withB. abortus . Surprisingly,B. abortus induced down-regulation of CD28 and up-regulation of B7.2 on murine CD4+ and CD8+ T cells. These effects on T cells were maximal for CD28 and B7.2 at 40 to 48 h and were not dependent on interleukin-12 (IL-12) or IFN-γ. On the basis of these results, we propose that the IL-12/IFN-γ-independent inhibition of Th2 responses to OVA/alum is secondary to the effects ofB. abortus on expression of costimulatory molecules on T cells. We suggest that down-regulation of CD28 following activation inhibits subsequent differentiation of Th0 into Th2 cells. In addition, decreased expression of CD28 and increased expression of B7.2 on T cells would favor B7.2 interaction with CTLA-4 on T cells, and this could provide a negative signal to developing Th2 cells.

Down-Regulation of Th2 Responses byBrucella abortus, a Strong Th1 Stimulus, Correlates with Alterations in the B7.2-CD28 Pathway