CD4 and granulomatosis with polyangiitis: Otherwise, on the basis of findings showing expression of IL-15 and MICA/MICB, presumably by macrophages and dendritic cells, in both renal and respiratory tract tissue of GPA, researchers have postulated that IL-15 together with MICA/MICB might be able to promote local CD4+ T cell proliferation via interaction with NKG2D-expressing cells [66,139].