Macrophages stimulated for tumouricidal activity show decreased MHC class II gene transcription and are generally poor presenters of antigen, despite their secretion of IL-1. Moreover, the process of tumour cell lysis is a multistep event, determined by the sensitivity of the target cell itself. Thus, activation to kill one target does not necessarily include the ability to kill all targets. Macrophages have receptors for this derivative, and it exerts additional activating effects on these cells, and perhaps some negative feedback on lymphocytes. This pathways is of some importance in man, since production of calcitriol can be so great that it leaks from the site of macrophage activation into the peripheral circulation, where it can exert its better known effects on calcium and phosphate balance.
Detectable hypercalcaemia can result.
There is also evidence that activated macrophages can be deactivated . This cytokine block activation by IFN- of increased capacity for production of ROI and, to some extent, of nitric oxide.