T cell recognition of isocitrate lyase in host immunity to tuberculosis

Abstract

Mycobacterium tuberculosis (MTB) is one of the world's most successful pathogens, infection two billion people and killing two million every year. The current vaccine, BCG, is of limited efficacy, and new vaccines are needed. The protein isocitrate lyase (ICL) of MTB is a member of the glyoxylate shunt and is required for the survival of MTB in immune-competent hosts. We hypothesized that the upregulation and requirement for continuous expression of ICL by MTB during chronic infection would make it an excellent target for the immune system. Using an interferon-gamma (IFNgamma) Elispot assay, T cell responses against the glyoxylate shunt enzymes ICL and malate synthase (MS) were detected in both humans and mice with latent M. tuberculosis infection. Responses to the cytoplasmic enzymes ICL and MS were similar in magnitude to those found against secreted or cell-wall MTB proteins known to be strong T cell antigens, such as ESAT-6 and Ag85A, confirming that proteins from bacterial cytoplasm can be processed and presented by antigen presenting cells during natural infection. Immunization of mice against ICL using both DNA- and protein-based vaccines stimulated the formation of ICL-specific CD4+ and CD8+ T cells, and provided partial protection against subsequent aerosol infection with M. tuberculosis . To enable the future study of T cell responses against ICL, CD4 and CD8 T cell epitopes of ICL were mapped using a peptide library consisting of 84 overlapping 16mer peptides covering the entire sequence of ICL. Minimal CD8 T cell epitopes presented in C57BL/6 and BALB/c mice were identified, and an epitope presented on the human MHC I molecule HLA-A2 was elucidated using HLA-A2 transgenic mice. CD4 T cell epitopes presented in C57BL/6, BALB/c, and C3H mice were also identified. This study demonstrates for the first time the recognition of the enzymes of the glyoxylate shunt by the immune response to M. tuberculosis, and demonstrates that intracellular proteins can provoke strong T cell responses that participate in protection against infection.