Cigarette smoking adversely affects the immune system, and is a risk factor for developing osteoporosis. How smoking contributes to osteoporosis is unclear, but since lymphocytes help maintain bone homeostasis and lymphocyte depletion results in bone loss, one potential mechanism for how smoke exposure promotes osteoporosis is by reducing bone marrow lymphocytes. Since the risk for developing osteoporosis is reportedly greater in smokers with polymorphisms in LRP5, a gene involved in canonical Wnt signaling that regulates bone metabolism, smoking-induced effects on lymphocytes may be influenced by Lrp5 functionality. To test these possibilities, we examined how the duration and cessation of cigarette smoke exposure affects lymphocyte distribution and function in normal mice and mice predisposed to low or high bone mass due to disruption or mutation of Lrp5. We find that, independent of genotype, mice exposed to cigarette smoke for 3-12 weeks showed a significant reduction in bone marrow B220(+)CD43(-) B cells and splenic transitional T1 B cells, and exhibited a splenic CD4(+):CD8(+) T-cell ratio that was skewed toward CD8(+) T cells. Smoke exposure had little or no effect on other lymphocyte subsets or on lymphocyte function ex vivo. Interestingly, these differences were no longer apparent after 6 weeks without smoke exposure, except in mice with high bone mass where bone marrow B220(+)CD43(-) B cells failed to fully recover. These data provide the first evidence that smoke exposure reduces bone marrow B cells, providing a plausible mechanism for how smoking contributes to osteoporosis.

Cigarette smoking is a risk factor for developing osteoporosis, and previous studies suggest lymphocytes are required for normal bone homeostasis and peak bone mass. We hypothesized that lymphocyte development is adversely affected by smoke exposure. If so, this outcome would provide a mechanism to partly explain how cigarette smoking promotes osteoporosis. To test this possibility, we investigated how the duration and cessation of cigarette smoke exposure affected the development, distribution, and function of lymphocytes normal mice. Consistent with the hypothesis, 3-12 weeks of smoke exposure significantly diminished bone marrow B220+CD43- B cell subsets, but, interestingly, did not significantly affect earlier progenitor B220+CD43+ B cells or thymic T cell subsets. In the spleen, by contrast, we find that individual splenic B cell populations, including transitional, marginal zone, and follicular mature B cell subsets, were largely unaffected by smoke exposure, but smoking did skew splenic CD4+:CD8+ T cell ratio to favor CD8+ T cells compared to sham controls. However, smoke exposure also had little or no effect on lymphocyte function ex vivo. Interestingly, smoking-induced alterations in lymphocyte populations observed at 12 weeks were lost when smoke exposure was stopped for six weeks. These data provide evidence that smoke exposure causes significant changes in lymphocyte populations that can largely be reversed after six weeks without smoking.