Innate Determinants of Microbial Immunity

National Institute Of Allergy And Infectious Diseases

Investigators

Linked publications, trials & patents

Abstract

(Progress on this project was limited because of our restricted laboratory access and the shortage of small animal care during the pandemic shutdown) Lung-resident myeloid cells are a heterogeneous population that occupy several distinct sub-tissular niches and perform diverse functions. Dendritic cells (DCs) and macrophages fulfil a tissue surveillance role, continually monitoring their environment for signs of infection or inflammation and providing the first line of defense. Recent reports have described heightened and long-lasting myeloid responsiveness following certain infections or vaccination procedures, a phenomenon termed as training or education. Such responses may be useful to induce in order to combat diseases with heavy myeloid cell involvement, such as tuberculosis. In work initiated last year, we evaluated the concept of immunization-induced myeloid education using a BCG vaccination model in which mice are immunized via the intravenous (iv) route previously shown to stimulate systemic inflammation and myelopoiesis in the bone marrow and a high level of host resistance against Mtb in a NHP model. We found that In contrast to conventional subcutaneous (sc) BCG immunization, iv inoculation dramatically altered the composition of the pulmonary myeloid cell compartment, triggering a striking, yet transient, depletion of Mtb-permissive alveolar macrophages (AM) In subsequent experiments performed during this report period we demonstrated that this depletion of AM is dependent on IFN-gamma producing T cells. In our previous work, we found that the empty alveolar niche in iv-vaccinated animals is subsequently re-populated by interferon-primed macrophages, likely derived from blood monocytes, that display heightened resistance to Mtb infection 5 months after vaccination and increased production of iNOS. Importantly, in new experiments performed in 2021 we showed that this heightened resistance is retained upon depletion of CD4+ T cells just prior to Mtb challenge. As reported previously, in addition to effects on macrophages, iv BCG preferentially promotes the development of IL-12-producing conventional XCR1+ DC1s. In the current report period we extended these observations by showing that this expansion results from the IFN-gamma dependent recruitment of DC1 committed bone precursors. As noted earlier such bone marrow-derived DCs (BMDCs) from iv-immunized animals were significantly more responsive to LPS or BCG stimulation than BMDCs generated from sc-immunized mice. Together, our previous and current results indicate that systemic BCG immunization has a profound and long-term impact on lung-resident macrophages and DCs, affecting both their population composition and responsiveness to microbial stimuli. Such remodeling of the lung myeloid compartment appears to be initially driven by CD4+T cell derived IFN-gamma but once established can be retained independent of CD4+T cell help. This process may contribute to the enhanced protection against Mtb reported in experimental animals immunized with BCG via the iv route, and as supported by the findings in the accompanying report can be potentially leveraged to protect against unrelated respiratory infections such as SARS CoV-2 We have previously demonstrated (Namasivayam et al, 2017, 2019) that anti-tuberculosis therapy (ATT) causes a distinct alteration in the composition of the intestinal microbiota that is long-lasting in both mice and humans and which could potentially impact host health. In this regard, it has been noted that individuals who have received ATT display an increased risk for re-infection and a previous clinical study using PBMCs reported that certain Mtb specific T cell epitopes were recognized poorly by individuals treated for TB in comparison to an untreated latently infected group. Interestingly, these same epitopes had higher homology to peptides identified in the microbiome (Scriba et al. Am J Respir Crit Care Med, 2017). Based on this evidence, it was hypothesized that depletion of microbiota taxa with peptides that cross-react with Mtb following ATT might affect the ability to maintain long-term resistance to TB. In work initiated during the review period we developed a mouse model to test this hypothesis. With Cecilia Lindestam Arlehamn at the La Jolla Institute of Immunology, we generated a pool of 288 peptides (MTB288) representing major TB antigen epitopes recognizable by mouse MHC. Of these, 68/288 peptides were found to have high sequence homology to microbiota epitopes (MTB68), whereas 130/288 peptides were specific to only Mtb (MTB130). Interestingly, following ex-vivo stimulation of cells isolated from Mtb-infected mouse lungs, the MTB288 as well as MTB68 pools elicited significantly higher T cell responses compared to the MTB130 pool. Moreover, several peptides in the MTB68 pool independently elicited a T cell response comparable to that observed following stimulation with an immunodominant epitope from ESAT-6, a peptide widely utilized to read out Mtb-specific T cell responses. Of note, none of the peptide pools including MTB68 displayed any T cell response in naive mice. Importantly, no reduction in the T cell response of infected mice to these epitopes was observed following ATT. Together these observations from our mouse model argue against the hypothesis that the increased susceptibility of previously antibiotic treated patients to Mtb infection is due to a diminished response to epitopes in the pathogen that cross-react with those in the microbiome. In a related COVID-19 project we asked whether the microbiome influences immune responses and host resistance to SARS-COV-2 in our mouse infection models. Following viral challenge mice treated with broad spectrum antibiotics to disrupt the microbiota showed no significant difference in survival, weight loss or virus titers but displayed a decreased CD8+ T cell response suggesting that the microbiome may influence the induction of adaptive immunity to secondary infection and possibly to COVID-19 vaccination