Development of Peripheral B and T Lymphocytes in Piglets

Maturation of the mitogen responsiveness, and IL2 and IL6 production by neonatal swine leukocytes

The maturation of the immune system of neonatal piglets was studied by following changes in the phenotypic composition and function of blood-borne leukocytes. The proportion of mature T and B lymphocytes decreased in the first week of birth and the circulating cells had poorly developed capacities to respond to mitogens and to secrete interleukins. From the end of the first week, however, there was a steady increase in the proportion of mature T cells (CD4+ and CD8+) and B cells in blood until 6-7 weeks after birth, when the study was ended. By 3-4 weeks, the relative proportions of different lymphocyte subsets resembled an adult-type composition. As they increased in prevalence, lymphocytes also developed capacities to proliferate and secrete interleukins. Proliferative responses to T-cell and B-cell mitogens reached adult levels within 2 weeks and 4-5 weeks, respectively. Blood leukocytes produced large quantities of IL6 by 1-2 weeks after birth and IL2 by 2-3 weeks. In contrast to lymphocyte patterns, the myeloid and granulocyte lineages were dominant at birth but then declined steadily. Unlike lymphocytes, the monocytes, macrophages and granulocytes appeared to be fully functional from the time of birth and exhibited a strong oxidative burst after appropriate stimulations. The magnitude of this response remained constant over the first 6-7 weeks. These results indicate that the first 3-4 weeks of post-natal life are a particularly susceptible interval for newborn piglets because constitutive and functional components necessary for specific cellular immune responses remain immature. This deficit may be offset by non-specific cellular mechanisms and maternally derived antibodies.