Strong suppression by monocytes of T cell mitogenesis in chicken peripheral blood leukocytes

Peripheral blood leukocytes (PBL) isolated from chicken blood by flotation on Ficoll-Hypaque (FH) have much lower proliferative responses to Con A and PHA than PBL isolated by slow-speed (SS) centrifuging at 60 X G. FH preparations contain all categories of blood cells except erythrocytes, whereas SS are almost devoid of nonlymphoid cells. FH responses approach SS levels after filtration through Sephadex G-10, which removes almost all monocytes detectable by neutral red and nonspecific esterase methods, while only partially depleting granulocytes and thrombocytes. Thus, the low responses of FH are probably associated with suppressor activity of monocytes in these preparations, numbering 6 to 8% of mononuclear cells. G-10 filtration decreases responses of SS preparations, indicating that the few (less than 0.1%) monocytes in SS function mainly as helper cells. In co-cultures, irradiated FH cells (FHX) but not SSX produce as much as 90% suppression of SS or FH responses. Suppression by FHX is totally inhibited by heat killing (56 degrees C for 45 min) or monocyte inactivation by preincubation with Trypan blue, and is removed by G-10 filtration, although not completely. Adherent cells isolated from unirradiated FH on Cytodex beads (ADC-CY) produce up to 99% suppression in co-culture with SS or nonadherent cells derived from FH. Soluble suppressor factors can be detected in conditioned media from supernatants of Con A-stimulated cocultures containing suppressor monocytes, but their suppressor activity is partially opposed by stimulatory factors, possibly interleukin 2, also present in supernatants. It is concluded that in FH preparations and therefore in blood, but not in SS, monocytes that have not been activated exert strong suppressor activity on mitogen-induced T cell proliferation. Occasional chickens of one inbred line, RPRL 72, had exceptionally high suppressor activity of monocytes, as shown by very low FH responses and very high suppressor effects of FHX well outside the normal range of variation found in this line or in line RPRL 63. This abnormal suppressor activity may have resulted from activation of suppressor monocytes as a response to subclinical infection.