Newborns and infants have limited IgG and IgA responses to infections and vaccines. In vitro studies have documented that the immunoglobulin class switch necessary for production of IgG and IgA requires both the expression of CD40 ligand (CD40L) on activated T-cells and the presence of interleukins. CD40L is expressed predominantly on CD4+ T-cells (helpers). Previous investigators have noted that activated neonatal T-cells express less CD40L than activated adult T-cells and have attributed the decreased production of IgG and IgA to this deficiency.

We have examined the expression of CD40L on activated T-cells from newborns, infants and adults to determine when adult levels of CD40L are expressed. Lymphocytes were activated with phorbol myristyl acetate and ionomycin for 4-16 hours and then examined by flow cytometry. Expression of CD69 (an early pan T-cell activation marker) documented appropriate T-cell activation. Cells were also stained with anti-CD3 and anti-CD8 (since CD4 is lost on activation) to allow determination of CD8-/CD3+ and CD8+/CD3+ cells.

We found levels of CD40L on activated umbilical cord blood lymphocytes which are identical with those seen in adults. No correlation between an increase in age and CD40L expression was noted. CD40L is an important member of the tumor necrosis factor family. We speculate that the monoclonal antibody used in these studies may detect other members of this family in addition to CD40L. Currently, a CD40 fusion protein, which was used by other investigators, is being utilized to study this further.