Pertussis toxin-induced lymphocytosis is associated with alterations in thymocyte subpopulations.

Pertussis toxin (Ptx), an important adjuvant for inducing certain organ-specific autoimmune diseases in mice, exerts multiple effects upon the immune system. In addition to its adjuvant effects, which include enhancement of delayed-type hypersensitivity and increased antibody production. Ptx elicits a marked lymphocytosis with a concomitant decrease in thymic weight. In vitro studies indicate that Ptx acts directly on thymocytes and that both susceptible and resistant populations exist. It is believed that these susceptible cells are released into the circulation and account, in part, for the T cell component of the lymphocytosis. We have used flow cytometry to analyze the CD4, CD8, and Thy-1 phenotypes of thymic and peripheral T cells from Ptx-treated mice. In the thymus, there is a dramatic decrease in the number of CD4+CD8+ (double positive) cells at all doses tested (0.25, 0.50, and 1.0 microgram) by day 4 after Ptx treatment. The double negative and single positive populations remain relatively constant. Analysis of Thy-1 expression reveals a significant reduction in Thy-1hi thymocytes, with little change in the Thy-1lo population. Thus Ptx primarily affects and depletes, in a dose-dependent fashion, thymic T cells with an immature phenotype. These results mimic those of corticosteroids, although neither prior adrenalectomy nor treatment with the antiglucocorticoid RU486 are able to prevent the effects of Ptx. In the periphery of Ptx-treated animals, the relative increase in the number of CD4+ T cells is more than that of CD8+ T cells. Double positive and Thy-1hi cells cannot be detected in appreciable numbers. These results are consistent with the concept that Ptx may drive immature thymocytes through accelerated maturation for release into the periphery as single positive, predominantly CD4+, Thy-1lo cells. Increased numbers of such cells may in part account for the immunopotentiating effects of Ptx, particularly as they relate to the induction of organ-specific autoimmune disease. Treatment with purified Ptx beta-oligomer fails to elicit any of the responses described above, indicating that the holotoxin is required for such activities.