CD4+ T Cells Recognizing PE/PPE Antigens Directly or via Cross Reactivity Are Protective against Pulmonary Mycobacterium tuberculosis Infection

Mycobacterium tuberculosis (Mtb), possesses at least three type VII secretion systems, ESX-1, -3 and -5 that are actively involved in pathogenesis and host-pathogen interaction. We recently showed that an attenuated Mtb vaccine candidate (Mtb Δppe25-pe19), which lacks the characteristic ESX-5-associated pe/ppe genes, but harbors all other components of the ESX-5 system, induces CD4⁺ T-cell immune responses against non-esx-5-associated PE/PPE protein homologs. These T cells strongly cross-recognize the missing esx-5-associated PE/PPE proteins. Here, we characterized the fine composition of the functional cross-reactive Th1 effector subsets specific to the shared PE/PPE epitopes in mice immunized with the Mtb Δppe25-pe19 vaccine candidate. We provide evidence that the Mtb Δppe25-pe19 strain, despite its significant attenuation, is comparable to the WT Mtb strain with regard to: (i) its antigenic repertoire related to the different ESX systems, (ii) the induced Th1 effector subset composition, (iii) the differentiation status of the Th1 cells induced, and (iv) its particular features at stimulating the innate immune response. Indeed, we found significant contribution of PE/PPE-specific Th1 effector cells in the protective immunity against pulmonary Mtb infection. These results offer detailed insights into the immune mechanisms underlying the remarkable protective efficacy of the live attenuated Mtb Δppe25-pe19 vaccine candidate, as well as the specific potential of PE/PPE proteins as protective immunogens.

Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis, is one of the most widely spread human pathogens, responsible for more than 9.6 million of new tuberculosis cases and 1.5 million deaths, annually. The resurgence of pulmonary tuberculosis in immuno-compromised patients, including HIV-co-infected populations, and increasing spread of drug-resistant Mtb strains are worrying. Given the estimated 2 billion cases of latent Mtb infections and the only partial efficacy of the unique, currently available tuberculosis-vaccine Mycobacterium bovis BCG (Bacille Calmette-Guerin) it is necessary to develop improved vaccines. Here, we demonstrate that the host cellular immunity, mediated by CD4⁺ T lymphocytes, specific to the “PE/PPE” families of mycobacterial antigens, contribute to the protection against Mtb-induced disease. We revealed the fine composition of the PE/PPE-specific T cells by characterizing their effector functions and differentiation status. We previously described a live attenuated mycobacterial strain as a vaccine candidate that is able to induce such CD4⁺ T cells and which displays particular properties at stimulating the cells of the innate immune system. These responses play a central role in the initiation of the host defense and in the protection against tuberculosis. Our results pave the way for further development of candidates in preclinical models of anti-tuberculosis vaccination.