Distinct IL-3 activation profile induced by intravenous versus subcutaneous routes of immunization

Lymphoid cells from mice immunized i.v. or s.c. with Leishmania major antigens were analyzed for their capacity to produce lymphokines when stimulated with specific antigens in vitro. Spleen cells from BALB/c mice immunized by the s.c. route produced significantly higher levels of IL-3 and IL-3 mRNA than those from mice immunized by the i.v. route. The differential production of IL-3 was maintained at a wide range of antigen concentrations tested in vitro and for different culturing times. T cell enrichment procedures and treatment with CD4+ mAb in vitro confirmed the T cell nature of the IL-3 producer population. However, the IL-3 production in the two populations of spleen cells was equally high after Con A stimulation in vitro. The IL-2 production by the two populations of cells was also not significantly different after antigen or mitogen stimulation in vitro. To our knowledge, this is the first report of a differential synthesis of IL-3 mRNA and secretion of IL-3 induced by different routes of immunization followed by specific-antigen stimulation in vitro. These findings may also explain earlier observations that i.v. immunization with leishmanial antigen induces protection, whereas s.c. immunization leads to exacerbation of L. major infection, since IL-3 has been previously shown to promote leishmanial infection. The fact that the phenomenon also extends to other antigen systems suggests that this finding may have a broader implication in immune regulation and vaccine development.