Differential expression of gamma interferon mRNA induced by attenuated and virulent mycobacterium tuberculosis in guinea pig cells after Mycobacterium bovis BCG vaccination

To determine whether Mycobacterium bovis BCG vaccination would alter gamma interferon (IFN-γ) mRNA expression in guinea pig cells exposed to Mycobacterium tuberculosis, we cloned a cDNA encoding guinea pig IFN-γ from a spleen cell cDNA library. The cDNA is composed of 1,110 bp, with an open reading frame encoding a 166-amino-acid protein which shows 56 and 41% amino acid sequence homology to human and mouse IFN-γ, respectively. Spleen or lymph node cells from naive and BCG-vaccinated guinea pigs were stimulated with purified protein derivative (PPD) or M. tuberculosis H37Ra or H37Rv, and the total RNA was subjected to Northern blot analysis with a ³²p-labeled probe derived from the cDNA clone. Compared to the IFN-γ mRNA expression in cells of naive animals, that in spleen and lymph node cells exposed to various stimuli was enhanced after BCG vaccination. However, there was a significant reduction in IFN-γ mRNA levels when cells were stimulated with a multiplicity of infection of greater than 1 virulent M. tuberculosis bacterium per 10 cells. The enhanced IFN-γ mRNA response in BCG-vaccinated animals was associated with an increase in the proportions of CD4⁺ T cells in the spleens, as determined by fluorescence-activated cell sorter analysis. Furthermore, the nonadherent population in the spleens enriched either by panning with anti-guinea pig immunoglobulin G-coated plates or by purification on nylon wool columns produced more IFN-γ mRNA than whole spleen cells following stimulation with concanavalin A or PPD. This indicates that T cells are principally responsible for the upregulation of IFN-γ mRNA expression following BCG vaccination. The mechanism by which virulent mycobacteria suppress IFN-γ mRNA accumulation is currently under investigation.