Down-regulation of Th2 responses by Brucella abortus, a strong Th1 stimulus, correlates with alterations in the B7.2-CD28 pathway

Down-regulation of the Th2-like response induced by ovalbumin-alum (OVA/alum) immunization by heat-killed Brucella abortus was not reversed by anti-IL-12 antibody treatment or in gamma interferon (IFN-gamma) knockout mice, suggesting that induction of Th1 cytokines was not the only mechanism involved in the B. 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 of B. abortus. B7.2, but not B7.1, was up-regulated on mouse non-T and T cells following immunization with B. 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-gamma. On the basis of these results, we propose that the IL-12/IFN-gamma-independent inhibition of Th2 responses to OVA/alum is secondary to the effects of B. 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.

Penicillin-induced dysfunction of platelet membrane glycoproteins

We examined the effects of the beta-lactam antibiotic penicillin G on platelet function and on specific membrane glycoproteins in vitro. Platelet concentrates exposed to 3 to 10 mmol/L penicillin for 48 hours showed irreversible inhibition of aggregation by thrombin in washed platelets after removal of the antibiotic. Although a brief 15-minute exposure to similar doses of penicillin also inhibited thrombin aggregation, the inhibition was reversed on removal of the penicillin by washing. Aggregation activity was also restored to normal levels by stimulation with high thrombin concentrations (> 0.4 U/ml). Results of the aggregation studies led us to examine how penicillin affects platelet membrane proteins. Membranes isolated after 48 hours of exposure of platelet concentrates to penicillin showed no differences from the control in total protein profiles on sodium dodecylsulfate--polyacrylamide gel electrophoresis or in the glycoprotein Ib or IIb content on immunoblotting. However, flow cytometric analysis with fluorescently labeled monoclonal antibodies revealed that exposure of platelets to penicillin for 15 minutes inhibited thrombin-induced modulations in the glycoproteins Ib, Ib-IX, IIb-IIIa, and P-selectin. These effects were observed with washed platelets and platelets in plasma. Penicillin also inhibited the regulation of expression of glycoproteins Ib-IX and IIb-IIIa in adenosine diphosphate--activated platelets. The inhibitory effects were partially reversed at high agonist concentrations.