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Department of Microbiology and Immunology, University of Miami Medical School, Miami, FL 33136
Recent studies have shown that neonatal mice are competent to
develop mature, Ag-specific Th1 function in situ. However, under many
conditions, Th2 responses dominate in the neonate, while Th1 responses
are more prevalent in adults. To compare further the immune responses
of neonates and adults, we used the enzyme-linked immunospot method to
measure the frequencies of primary Th1/Th2 effectors generated in situ
in the spleens and lymph nodes. As assessed by the detection of
IFN-
- or IL-4-producing cells, adults developed mixed Th1/Th2
responses in both organs. Neonatal lymph nodes contained mature
frequencies of IFN-- and IL-4-producing cells. In striking contrast,
while mature frequencies of Th2 cells developed in neonatal spleens,
virtually no IFN--secreting cells were detected. Exclusive Th2
function was observed in both BALB/c and C57BL/6 neonates, strains in
which the Th2 and Th1 lineages, respectively, are favored in adults.
Although Th1 effectors were virtually undetectable, the addition of
rIL-12 boosted the frequency of IFN--secreting cells to adult
levels. Therefore, Th1 effectors apparently developed in situ, but Th1
effector function either was not promoted or was inhibited upon
subsequent exposure to the Ag in culture. Together, these results
indicate that the quality of a primary Th response in neonates is
strongly dependent on the site of initial Ag exposure; responses
initiated in the lymph nodes are mixed Th1/Th2, whereas responses
occurring in the spleen are heavily Th2 biased.
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