While this mechanism of action for vitamin D was demonstrated in vitro, little clinical work has been done to validate this piece of knowledge in clinical practice and in the care of TB patients. As proxy for in vivo studies, subsequent experiments by Liu et al evaluated induction of cathelicidin by TLR activation of macrophages in sera from African American and Caucasian subjects. These experiments demonstrated failure to induce detectable levels of cathelicidin mRNA in samples from African American patients, who are known to be significantly more vitamin D deficient than Caucasians due to difference in vitamin D production in the skin. The difference in cathelicidin mRNA levels was corrected upon supplementation of deficient sera to therapeutic levels of 25(OH)D3 prior to TLR stimulation.