TNF-α-Dependent ICAM-1- and VCAM-1-Mediated Inflammatory Responses Are Delayed in Neonatal Mice Infected with Pneumocystis carinii

Neonatal mice have a delayed CD4-mediated inflammatory response to Pneumocystis carinii (PC) infection in the lungs that corresponds to a delayed TNF-α response and a delayed clearance of the organisms compared with adult mice. Since TNF-α is known to drive the up-regulation of adhesion molecules, we examined the expression and function of adhesion molecules in the lungs of neonatal mice. The expression of both ICAM-1 and VCAM-1 was significantly lower in the lungs of PC-infected neonatal mice compared with adults. Additionally, migration of neonatal T cells across endothelial cells expressing VCAM-1 and monocyte chemotactic protein-1 was aberrant compared with that in adult T cells, although α₄β₁ integrin-mediated adhesion of neonatal lymphocytes was comparable to that of adult lymphocytes. Treatment of neonatal mice with exogenous TNF-α resulted in increased expression of ICAM-1 and VCAM-1 as well as increased expression of chemokines, resulting in infiltration of CD4⁺ cells into the lungs. Treatment with exogenous TNF-α resulted in a trend (although not statistically significant) toward a reduction of PC organisms from the lungs. These data indicate that neonatal lung endothelial cells do not up-regulate ICAM-1 and VCAM-1 in response to PC infection, probably due to depressed TNF-α production. Additionally, neonatal T cells are defective in the ability to migrate across endothelial cells.