The role of interleukin-12 in human infectious diseases: only a faint signature

IL-12 is the signature IFN-gamma-inducing cytokine and, as such, is thought to be crucial for protective immunity against intracellular microorganisms. This concept is supported by results from experimental infections of knockout mice lacking IL-12 or the IL-12 receptor. The description of human patients with inherited IL-12 or IL-12-receptor deficiency challenges this view. Indeed, in natural conditions of infection and immunity - the hallmark of the human model - IL-12 was found to be redundant in defense against intracellular microorganisms other than Mycobacteria and Salmonella. More surprisingly, IL-12 was recently found to be redundant even in defense against primary intection by Mycobacteria and Salmonella in many patients, and against secondary infection by Mycobacteria but not Salmonella in most patients.