ESAT-6 Targeting to DEC205+ Antigen Presenting Cells Induces Specific-T Cell Responses against ESAT-6 and Reduces Pulmonary Infection with Virulent Mycobacterium tuberculosis

Abstract

Airways infection with Mycobacterium tuberculosis (Mtb) is contained mostly by T cell responses, however, Mtb has developed evasion mechanisms which affect antigen presenting cell (APC) maturation/recruitment delaying the onset of Ag-specific T cell responses. Hypothetically, bypassing the natural infection routes by delivering antigens directly to APCs may overcome the pathogen’s naturally evolved evasion mechanisms, thus facilitating the induction of protective immune responses. We generated a murine monoclonal fusion antibody (α-DEC-ESAT) to deliver Early Secretory Antigen Target (ESAT)-6 directly to DEC205+ APCs and to assess its in vivo effects on protection associated responses (IFN-γ production, in vivo CTL killing, and pulmonary mycobacterial load). Treatment with α-DEC-ESAT alone induced ESAT-6-specific IFN-γ producing CD4+ T cells and prime-boost immunization prior to Mtb infection resulted in early influx (d14 post-infection) and increased IFN-γ+ production by specific T cells in the lungs, compared to scarce IFN-γ production in control mice. In vivo CTL killing was quantified in relevant tissues upon transferring target cells loaded with mycobacterial antigens. During infection, α-DEC-ESAT-treated mice showed increased target cell killing in the lungs, where histology revealed cellular infiltrate and considerably reduced bacterial burden. Targeting the mycobacterial antigen ESAT-6 to DEC205+ APCs before infection expands specific T cell clones responsible for early T cell responses (IFN-γ production and CTL activity) and substantially reduces lung bacterial burden. Delivering mycobacterial antigens directly to APCs provides a unique approach to study in vivo the role of APCs and specific T cell responses to assess their potential anti-mycobacterial functions.

Two distinct conformational states of Mycobacterium tuberculosis virulent factor early secreted antigenic target 6 kDa are behind the discrepancy around its biological functions

“…We found that the ASB nontreated preparation but not the ASB‐treated one was able to induce higher level of IFN‐γ secretion. Interestingly, we noted that studies reporting the induction of IFN‐γ secretion by ESAT‐6 either used the form bound to CFP‐10, purified from Mtb cell culture supernatant , or ESAT‐6‐derived small peptides . Other groups using recombinant forms of ESAT‐6, without adding a washing step with ASB to their purification protocols, also found that such a form did induce the secretion of IFN‐γ by TB patientT cells .…”

Targeting dendritic cells to accelerate T-cell activation overcomes a bottleneck in tuberculosis vaccine efficacy

“…Previous studies have attempted to improve DC function at the time of vaccination by targeting the CD40 activation pathway49, DC receptor Dec-205 (refs 50, 51), or by utilizing cell-derived vaccines, such as Mtb antigen-primed DCs3132. Improving DC function by targeting the CD40 pathway during vaccination had no effect49, while activating Dec-205 at the time of vaccination had a small effect (∼0.5 log reduction) on vaccine-induced protection on Mtb challenge5051.…”

“…Improving DC function by targeting the CD40 pathway during vaccination had no effect49, while activating Dec-205 at the time of vaccination had a small effect (∼0.5 log reduction) on vaccine-induced protection on Mtb challenge5051. Vaccination with antigen-primed DCs has generated mixed results in Mtb -infected mice, with a study demonstrating a negative impact of DC vaccination due to exuberant inflammation31, or induction of vaccine-induced protection similar to levels seen in BCG-vaccinated hosts32.…”

ESAT-6–dependent cytosolic pattern recognition drives noncognate tuberculosis control in vivo

“…Therefore, determining which human APC subset in different tissues is the most efficient one for sensing ESAT-6 might enable the development of improved strategies aimed at the targeted delivery of TB vaccines to a particular APC subset (73,74) and lead to optimal and localized IL-18-mediated IFN-γ secretion. Another critical question arises from our study: how does ESAT-6 activate NLRP3 in vivo?…”