No abstract

These results, together with our previous finding that intravenous administration of MRK-16 induced regression of multidrug-resistant subcutaneous tumors in athymic mice, support the hypothesis that the combined use of MRK-16 and cyclosporine might increase the efficacy of antitumor agents against multidrug-resistant tumors expressing P-glycoprotein. Clinical phase I trials of MRK-16 in the treatment of multidrug-resistant tumors are under consideration.

SDZ PSC833 (PSC833).

The synergistic effect of MRK‐16, a monoclonal antibody against P‐glycoprotein, and cyclosporin A (CsA) on the modulation of vincristine resistance was studied by isobologram analysis in three different, highly multidrug‐rcsistant tumor cells. In all cell lines, the synergistic effect was demonstrated at various concentrations of MRK‐16 and CsA. While MRK‐16 alone did not enhance the sensitivity of the moderately resistant KB‐8–5 cells to vincristine, it increased two‐fold the reversing effect of cyclosporin A at 1 μM, an achievable blood concentration. Since MRK‐16 alone showed therapeutic effects against multidrug‐resistant tumors, the combined use of MRK‐16, CsA and antitumor agents should provide therapeutic benefits for the treatment of resistant tumors.

Follicular helper T (Tfh) cells are crucial for humoral immunity. Dysregulation of Tfh cell differentiation can cause infectious, allergic, and autoimmune diseases. To elucidate the molecular mechanisms underlying Tfh cell differentiation, we attempted to establish an in vitro mouse model of Tfh cell differentiation in the absence of other cell types. Various cytokines and cell surface molecules are suggested to contribute to the differentiation. We found that stimulating naïve CD4+ T cells with immobilized antibodies to CD3, ICOS, and LFA-1 in the presence of soluble anti-CD28 antibody, IL-6, and antibodies that block IL-2 signaling for 3 days induced the expression of Bcl6 and Rorc(γt), master regulator genes of Tfh and Th17 cells, respectively. TGF-β significantly enhanced cell proliferation and Bcl6 and Rorc(γt) expression. An additional 2 days of culture without immobilized antibodies selectively downregulated Rorc(γt) expression. These cells produced IL-21 and promoted B cells to produce IgG antibodies. Adding the aryl hydrocarbon receptor (AhR) antagonist CH-223191 to the T cell culture further downregulated Rorc(γt) expression without significantly affecting Bcl6 expression, and upregulated expression of a key Tfh marker, CXCR5. Although their CXCR5 expression levels were still not high, the CH-223191-treated cells showed chemotactic activity towards the CXCR5 ligand CXCL13. On the other hand, AhR agonists upregulated Rorc(γt) expression and downregulated CXCR5 expression. These findings suggest that AhR activity and the duration of T cell receptor stimulation contribute to regulating the balance between Tfh and Th17 cell differentiation. Although this in vitro system needs to be further improved, it may be useful for elucidating the mechanisms of Tfh cell differentiation as well as for screening physiological or pharmacological factors that affect Tfh cell differentiation including CXCR5 expression.