IL17A and Granuloma: IDO-1 expression in granulomas is found mainly in the rim that surrounds the central necrosis (270). IDO-1 catabolizes tryptophan into kynurenine. Since tryptophan is normally required for Mtb growth, absence of IDO-1 would theoretically provide excess fuel for Mtb growth (271). Insufficient IDO-1 would increase IL-17, resulting in increased neutrophil influx into and cause necrosis of the granulomas (Figure 7).