Constitutive expression of transgenic heat stable antigen (mCD24) in lymphocytes can augment a secondary antibody response.

The murine heat stable antigen (HSA, mouse CD24) is a glycosyl phosphatidylinositol-anchored cell surface protein which is primarily expressed in immature but not mature cells of several hematopoietic lineages and in neuronal tissue. The function of HSA is not known but there is evidence in lymphocytes that it is involved in cell adhesion and in cell activation. We examined the effect of constitutive HSA expression on the maturation and on the function of B and T cells in transgenic mice. Transgenic HSA was strongly expressed throughout all stages of T cell maturation without changes in absolute cell number or proportions of subpopulations both in the thymus and in the periphery. The size of the B cell compartment was also unchanged. Thus, we conclude that in the T lineage the loss of HSA expression is not mandatory for maturation. On the functional level, two parameters of immune function were measured. When the ability to activate peripheral T cells expressing transgenic HSA was tested in a mixed lymphocyte reaction, no significant difference in the stimulation index and cytolytic activity was detected between transgenics and control littermates. However, when immunized with a T cell dependent antigen, transgenic mice showed 10-fold higher serum IgG1 titers. This suggests that the expression of transgenic HSA in cells normally negative for HSA (e.g. peripheral T cells or memory B cells) leads to a better stimulation of lymphocytes during a secondary antibody response.