TBXT and Granuloma: Interestingly, any of these strains that were also deficient in T-bet, a transcription factor that is critical for Th1 cell differentiation (26), were susceptible to M. tuberculosis infection, as shown by bacterial counts (Figs. 1, A, and 2, A and E) and the presence of necrotic granulomas (Figs. 1B, and 2, B and F).