Susceptibility of mice deficient in the MHC class II transactivator to infection with Mycobacterium tuberculosis.

Major histocompatibility complex (MHC) class II antigen presentation and subsequent CD4+ T-cell activation are critical for acquired immunity to Mycobacterium tuberculosis infection. MHC class II gene expression is primarily controlled by the master transactivator CIITA protein. Without functional CIITA protein, MHC class II expression is lost, impairing immune responses and increasing susceptibility to infection. In this study, we compared protective immune responses of CIITA-deficient mice and wild-type C57BL/6 controls with low dose aerosol M. tuberculosis infection. After aerogenic challenge, CIITA-/- mice failed to limit mycobacterial growth (2.5 and 2.0 log10 > WT lung and spleen CFUs, respectively, at day 58). Lung histopathology involved extensive necrosis, severe pneumonitis and overwhelming inflammation in the gene knockout mice. Mean survival time for CIITA-/- mice was significantly reduced (57 versus >300 days for WT). This extreme sensitivity to tuberculous infection was largely attributed to the absence of CD4+ cells. Flow cytometric studies detected virtually no CD4+ cells in CIITA-/- mouse spleens after infection versus elevated numbers in WT spleens. Failed CD4+ T-cell expansion markedly reduced interferon-gamma (IFN-gamma production in CIITA-/- mice versus WT controls. These results suggest the necessity of a functional CIITA pathway for controlling tuberculous infections and that interventions targeting CIITA expression may be useful antimycobacterial therapeutics.

Immunosuppression derived from human B-lymphoblastoid and melanoma cell lines.

Previous work conducted by the authors, using a murine model, suggested that soluble factors secreted by tumor cells suppress lymphocyte responses. To apply this premise to human tumors, the effects of UC729-6 (lymphoblastoid B-cell) and M21-HPB (malignant melanoma) conditioned media (CM) on normal lymphocyte proliferation, as well as on tumor cell growth in autologous CM was studied. The CM was collected at 2-5 day intervals from cultures of UC729-6 and M21-HPB cells in serum-free media. Phytohemagglutinin- and concanavalin A-stimulated mononuclear peripheral blood cells from healthy human donors showed decreased [3H]thymidine ([3H]Tdr) uptake in the presence of each CM when compared with controls. In assays using 100% CM, mitogen stimulation was 68-85% less than that of controls and 40-50% less using 50% CM. The suppression was more pronounced with UC729-6 CM than with M21-HPB CM. In mixed lymphocyte cultures (MLC), addition of 50% CM from either tumor cell line resulted in 40-50% reduction in [3H]Tdr uptake by lymphocytes. Incubation of UC729-6 cells in 5% to 100% of UC729-6 fCM (filter-concentrated) produced a decrease in [3H]Tdr uptake which was directly proportional to the amount of fCM present. In contrast, M21-HPB cell growth in autologous fCM was dependent on cell number, as well as on the amount of fCM used. Treatment of the UC729-6 fCM using acid (pH 4.5), trypsin (100 micrograms/ml), and heat (56 degrees C) did not restore mitogen-stimulated lymphoproliferation. However, the inhibition observed with UC729-6 fCM was partially reversed after dialysis with membranes having M(r) limits of 2.5 x 10(4), 1.5 x 10(4), or 1 x 10(4).