Vitamin D critical to TB immune response

According to researchers at the University of California - Los Angeles, effective immune responses to tuberculosis infection are critically dependent on the availability of vitamin D.
During a healthy response to an invasion by the TB-causing Mycobacterium tuberculosis, macrophages mount a frontal assault on the pathogens, but the process failed to happen in vitro when the human macrophages were incubated in serum with low levels of vitamin D. When the serum was supplemented with 25-hydroxyvitamin D3, antimicrobial activity in the macrophages was restored, MedPage Today reports.
"The present findings underscore the importance of adequate amounts of vitamin D in all human populations for sustaining both innate and acquired immunity against infection," Robert Modlin and his colleagues wrote, according to MedPage Today.
Subsequent research found that innate immune responses, mediated by Toll-like receptors, were able to stimulate macrophages to attack cells infected with the bacteria as long as vitamin D was available.
Because dark skin reduces the ability to make vitamin D from sun exposure, this may explain why tuberculosis and other infections are more common in dark-skinned populations.  
"Vitamin D deficiency may compromise both the innate and the acquired antimicrobial host defense pathways against tuberculosis infection and likely other infections known to be greater in blacks," the researchers wrote, according to MedPage Today.
Modlin and his colleagues stressed that the role of vitamin D immune responsiveness must be investigated in human cells and tissues, not in models using mice.
"The interferon-gamma-induced antimicrobial pathway described here is vitamin D-dependent in humans but not in the mouse," the authors wrote, MedPage Today reports. "The evolution of distinct antimicrobial mechanisms makes sense teleologically as well because mice are nocturnal animals and humans are not, and the amount of vitamin D increases with sun exposure."