It is now recognized that reproductive hormones such as estrogen influence not only classical targets (eg. reproductive tissues), but may also act on non-classical target sites such as the immune system. A better understanding of the effects of estrogen on the immune system is of paramount importance since: (i) increasing numbers of women around the world take estrogen-containing oral contraceptives, some times for most of their reproductive life; (ii) estrogen is often prescribed as a replacement therapy to postmenopausal women; and (iii) a large number of pesticides, insecticides, and phytoestrogens (plant-derived estrogens) have been found to have hormone disrupting effects as evidenced by altered development of reproductive and immune functions in wild species. The precise effects of estrogen on the normal immune system are not well known. The overall objective of this work has been to better understand the role of sex hormones on the B cell function of normal mice. It is hoped that this will lead to an improved understanding of the pathogenesis and treatment of immune-related disorders such as autoimmune diseases and cancer. These studies were accomplished by parenteral administration of estrogen to nonautoimmune C57BL/6 mice, a widely used strain in immunology. In these mice, we found that treatment with estrogen, but not 5a-dihydrotestosterone. induced the expression of a wide variety of IgG and IgM autoantibodies and heteroantibodies that are associated with autoimmune and infectious diseases. These include antibodies to cardiolipin and other membrane phospholipids, dsDNA and acetone-killed Brucella abortus strain RB51. Importantly, the expression of anti-dsDNA and anti-cardiolipin antibodies was sustained for several months after the removal of the exogenous source of estrogen. This indicates that the immunomodulatory effect of estrogen is long-lasting. These antibodies have a marginal degree of crossreactivity with other antigens and belong mainly to IgG2b subisotype. These findings were confirmed at the cellular level, where we have shown that estrogen-treated mice have increased numbers of plasma cells in the spleen, and that these plasma cells actively secrete IgM and IgG immunoglobulins as assessed by ELISPOT. Further, higher immunoglobulin yield per cell was evident in estrogen-treated than in placebo-treated controls in the spleen and bone marrow. Interestingly, we found that splenic lymphocytes had an increase in antibody-forming cells for all specificities tested. Active antibody-forming cells from bone marrow preferentially recognized autoantigens, cardiolipin and dsDNA. Functional analysis on the viability of the splenic lymphocytes showed that in vivo exposure to estrogen resulted in: (a) increase in the proportion of cells dying by apoptosis, and (b) an increased proportion of lymphocytes that were actively proliferating as assessed by cell cycle analysis. Culturing of B cells in the absence of any deliberate stimulus showed the B cells from estrogen-treated mice underwent active proliferation and resisted death by apoptosis more compared to controls. We also found that despite the autoproliferative character of splenic B cells, they were able to respond adequately to stimulation with anti-CD40 antibodies, IL-4 and lipopolysaccharides. B cells from mice treated with estrogen had a marked reduction in their susceptibility to apoptosis when cultured in the presence of such stimuli. Together these studies indicate that normal mice exposed to estrogen may express a variety of autoantibodies, show signs of B cell hyperactivity, have defects in susceptibility of B cells to apoptosis as well as the ability to proliferate in the absence of stimulation. It is hoped that these studies would enhance our understanding of the immunomodulatory role of estrogen in health and in a wide range of disorders such as autoimmune and cancer disorders. / Ph. D.