One proposed pathway for this observed immunomodulatory effect of vitamin D in TB infection involves the toll-like receptor (TLR) signaling pathways of the innate immune system. Liu et al demonstrated that macrophage TLR binding of M. tuberculosis antigen upregulates the expression of cellular 1 hydroxylase, resulting in higher levels of active 1, 25(OH)2D. It was also shown that 1, 25(OH)2D in turn binds the nuclear vitamin D receptor (VDR), which activates a downstream signaling cascade resulting in the upregulation of several aspects of the innate immune response, including the induction of a newly described anti-microbial peptide LL-37, a cathelicidin. Cathelicidins, in addition to their broad spectrum antimicrobial activity, also possess chemotactic properties for neutrophils, monocytes and T cells, among other functions within the immune system.