title: Suppression of B cell function by methotrexate and trimetrexate. Evidence for inhibition of purine biosynthesis as a major mechanism of action.
abstract: 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.
authors.affiliation: Systemic Toxicology Branch, National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709.
authors.forename: G J
authors.initials: GJ
authors.lastname: Rosenthal
authors.forename: G W
authors.initials: GW
authors.lastname: Weigand
authors.forename: D R
authors.initials: DR
authors.lastname: Germolec
authors.forename: J A
authors.initials: JA
authors.lastname: Blank
authors.forename: M I
authors.initials: MI
authors.lastname: Luster
mesh_terms: D000818:Animals; D000917:Antibody Formation / Q000187:drug effects; D001402:B-Lymphocytes / Q000187:drug effects* / Q000276:immunology / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D005260:Female; D005493:Folic Acid Antagonists / Q000494:pharmacology; D007136:Immunoglobulins / Q000096:biosynthesis; D007166:Immunosuppressive Agents / Q000494:pharmacology*; D007700:Kinetics; D008213:Lymphocyte Activation / Q000187:drug effects; D008727:Methotrexate / Q000494:pharmacology*; D051379:Mice; D011687:Purines / Q000037:antagonists & inhibitors* / Q000096:biosynthesis; D011799:Quinazolines / Q000494:pharmacology*; D016597:Trimetrexate
chemical_list: D005493:Folic Acid Antagonists; D007136:Immunoglobulins; D007166:Immunosuppressive Agents; D011687:Purines; D011799:Quinazolines; D016597:Trimetrexate; C030985:purine; D008727:Methotrexate