In vivo induction of CD4+ T cell responses by antigens covalently linked to synthetic microspheres does not require adjuvant.

Macrophages, dendritic cells or B lymphocytes have been shown to play a major role in the presentation of soluble antigens to CD4+ T cells. In contrast, the capacity of these cells to present particulate antigens such as bacterial or parasitic antigens to T cells remains controversial. To investigate this question, well defined particulate antigens were prepared by covalent linkage of proteins or peptides to 1 micron in diameter synthetic microspheres. The T cell immunogenicity of such particulate antigens was analyzed in vitro and in vivo. In vitro, a soluble protein such as hen egg lysozyme (HEL) coupled to beads stimulated a strong proliferative T cell response of lymph node cells from HEL-primed mice or of specific T cell hybridomas. HEL coupled to beads was presented to the specific T cell hybridomas by splenocytes or by peritoneal macrophages, but not by lymphoma B cells. Immunization of mice with several different protein antigens or with a synthetic peptide covalently linked to beads induced strong CD4+ T cell responses in the absence of adjuvant. The strong in vivo immunogenicity of proteins coupled to beads did not result from a non-specific adjuvant effect of beads since covalent linkage of the antigen to beads was strictly required to induce T cell responses in the absence of adjuvant. In vivo treatment by carrageenan showed that macrophages are required for the in vivo stimulation of T cell responses by these particulate antigens. Thus, these results demonstrated the role of phagocytic cells, especially macrophages, for in vivo presentation of particulate antigens. These particulate antigens represent an interesting approach for the development of new vaccines, and for the in vivo analysis of the role of various antigen presenting cells in T cell activation and differentiation.