Age-dependent immune response in Merino sheep.

The numerical and functional attributes of populations of lymphocytes were compared in the blood, lymph and skin of young and mature sheep. Young sheep, four to eight months old, had a lower proportion of CD4+ cells in blood, lymph and skin than mature sheep three to six years old. In contrast, B cells and T19+ cells were as prevalent or more prevalent in young sheep as in mature sheep. Blood lymphocytes from young sheep, cultured in vitro produced less interferon-gamma, both spontaneously and in the presence of concanavalin A than did lymphocytes from older sheep. The serum antibody responses of adult sheep to the T cell-independent antigen Brucella abortus lipopolysaccharide (LPS) were greater over a range of antigen doses, suggesting that an apparent excess of antigen could not overcome the relative immune deficiency of young sheep. The adjuvant Quil A corrected the depressed antibody response of young sheep to B abortus LPS, but dextran sulphate did not. The skin contact hypersensitivity of mature sheep to dinitrochlorobenzene was greater. However, the T cell phenotypes present in infiltrates of lymphocytes elicited by the intradermal injection of tetanus and diphtheria, but not tuberculin antigens, were comparable for the two age groups. The capacity of Quil A to raise the antibody responses of both young and mature sheep to a similar titre suggests that it may be possible to overcome the immunological hyporesponsiveness that may contribute to the disease susceptibility of young sheep.