Feline leukemia virus infection downmodulates the production of growth-inhibitory activity by marrow stromal cells.

To determine the role of infected marrow accessory cells in the pathogenesis of viral-associated hematologic disorders, we evaluated whether feline leukemia virus (FeLV) infection alters the cytoadhesive properties of long-term marrow culture (LTMC) stromal cells, the support of stromal-associated progenitors in LTMCs, and the production of progenitor growth-promoting and -inhibiting activities by marrow stromal cells. Our previous studies demonstrated that LTMCs containing FeLV-infected stromal cells generated two- to three-fold higher numbers of total nonadherent cells and nonadherent granulocyte-macrophage progenitors (CFU-GM) compared with uninfected LTMCs. In the present studies, CFU-GM and primitive erythroid progenitors (BFU-E) bound equivalently to FeLV-infected or uninfected LTMC stromal cells in a 2-hour adherence assay. In recharge LTMC studies, the numbers of adherent CFU-GM maintained in cultures containing stromal cells infected with FeLV-A/61E were not significantly different from controls (range 84-191% of uninfected control cultures, p > 0.1); however, the percentages of adherent CFU-GM in S phase of the cell cycle were consistently increased (range 42-62% compared with controls, range 5-23%). FeLV infection had no significant effect on the cell-cycle status of the nonadherent CFU-GM in LTMCs. Agar co-culture assays revealed that multilineage colony-stimulating activity was constitutively and equivalently produced by feeder cell layers consisting of either uninfected or FeLV-infected irradiated heterogeneous LTMC stromal cells, homogeneous marrow stromal fibroblasts, or a fibroendothelial marrow stromal cell line. However, FeLV infection significantly attenuated the soluble progenitor growth-inhibitory activity associated with higher densities of these stromal cells. Assays of conditioned medium from cultures of irradiated stromal cells demonstrated that FeLV infection or hydrocortisone exposure decreased the utilization of glucose, the production of acidic metabolic products, and the constitutive production of active and latent transforming growth factor beta (TGF-beta) bioactivity and TGF-beta 2 immunoreactivity. Levels of macrophage inflammatory protein 1 alpha (MIP-1 alpha) and tumor necrosis factor alpha (TNF-alpha) were undetectable and unchanged in CM samples. Together, these observations suggest that downmodulation of TGF-alpha and/or the basal metabolic status of stromal cells may be responsible for the high basal proliferative activity of adherent CFU-GM in FeLV-infected LTMCs, and by extension, that retroviral infection in vivo could alter hematopoiesis by perturbing the progenitor growth-regulatory and -supportive function of marrow stromal cells.