Suppression of B cell function by methotrexate and trimetrexate. Evidence for inhibition of purine biosynthesis as a major mechanism of action.

Methotrexate (MTX) is a widely used drug in the treatment of a variety of human neoplasms. Trimetrexate (TMQ) is a lipid-soluble quinazoline derivate of MTX that, unlike MTX, is not dependent upon membrane folate transport for cellular entry. A number of studies have demonstrated that MTX and, more recently, TMQ possess potent immunosuppressive properties. To examine the cellular events associated with the immunomodulatory effects of anti-folates on humoral immunity, a murine B cell maturation model was used. In vitro, MTX and TMQ reduced the number of antibody-forming cells to SRBC, as well as IgM production. B cells stimulated with anti-Ig demonstrated a dose-related suppression in [3H]UdR incorporation after addition of either drug, suggestive of a decrease in de novo DNA synthesis. B cell activation events preceding S phase were also suppressed by both anti-folates, as evidenced by inhibition of RNA synthesis. However, neither drug affected surface expression of Ia Ag nor inositol phosphate accumulation. Addition of TdR caused a slight non-significant increase in the antibody-forming cell response in the presence of 10(-7) M MTX. However, addition of hypoxanthine or adenine, but not guanine, resulted in complete restoration. Timed addition revealed that the ability of MTX to suppress antibody responses was diminished if added after 48 h of culture, similar to the reversal of this suppression mediated by hypoxanthine. Cell cycle analysis of LPS-stimulated B lymphocytes demonstrated that both drugs modulated events preceding, as well as during, the S phase. The present studies suggest that although drug-induced impairments in dTMP biosynthesis may be responsible for deficient lymphoid proliferation, anti-folate-induced impairment in purine biosynthesis is a major mechanism in anti-folate-induced suppression of humoral immunity.