Signaling through Axl receptor plays a host-protective role during experimental tuberculosis

Author(s)

André Aparecido dos Santos Correa

• Document type: Master's Dissertation
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Medicina de Ribeirão Preto (PCARP/BC)
• Defense date: 2022-12-16

Examining board members

Diego Luís Costa; Larissa Dias da Cunha; Alessandra D'Almeida Filardy

• Advisor: Diego Luís Costa

Abstract

Tuberculosis (TB), caused by Mycobacterium tuberculosis (Mtb) bacilli is the infectious disease that killed most people in the last decade. Strategies aimed at modulating host immune responses have been proposed as promising adjuvant therapies to antibiotics, with the aim of optimizing the TB treatment of TB. Axl and MerTK receptors recognize phosphatidylserine bound to Gas6 on the surface of apoptotic cells and trigger intracellular anti-inflammatory and immunosuppressive signaling. The role of these receptors in the pathogenesis of TB is unknown, however, considering their immunoregulatory potential, it is possible that the characterization of their role during Mtb infection identifies this pathway as a potential target for immunopharmacological interventions with the aim of optimizing the therapy for TB. The aim of this project was to characterize the role played by Axl and MerTK receptors during experimental TB. Initially, we observed that the infection of macrophages with Mtb H37Rv induces a relevant increase in the expression of Axl, but not of MerTK in these cells. This increase was partially dependent on the expression of ESAT6 by the bacteria and on the activation of the Toll Like Receptor 2 (TLR2) in the infected macrophages. In vivo, we observed that Mtb infection induces increased expression of Axl and MerTK mainly in inflammatory macrophages, and that the ligand Gas6 and soluble forms of the receptors also accumulate in the lungs of infected animals. To assess the role of Axl and MerTK in the resistance to Mtb infection, we initially evaluated whether receptor deficiency alters resistance to Mtb infection in macrophages in vitro. We observed that Axl-/- and MerTK-/- cells control intracellular bacterial replication more efficiently compared to C57BL/6 cells in the absence of stimuli, but in the presence of IFN-γ activation there is no difference between the bacterial load of C57BL/6, Axl-/- or MerTK-/- cells, although the production of nitric oxide by activated Axl-/- macrophages was higher than that of C57BL/6 and MerTK- /- cells. In vivo, we observed that Axl-/- mice exhibited increased mortality compared to C57BL/6 and MerTK-/- mice after infection with high dose of Mtb. After 3 weeks of infection, we observed mainly a higher infiltration of neutrophils in the lungs of Axl-/- mice compared to the other groups. Additionally, the levels of IL1β and TNF in lung homogenates, as well as the production of IFN-γ and TNF-α by CD4+ T cells in Axl-/- mice were higher compared to the other groups. There was, however, no difference in pulmonary bacterial loads among the groups. In parallel, we observed that the pharmacological blockade of Axl resulted in higher susceptibility to experimental TB, since these animals became moribund 24 days after infection, a phenomenon that was not observed in mice treated with vehicle or MerTK inhibitor. Mortality was not associated with uncontrolled replication of the pathogen, since the pulmonary bacterial load was similar in animals untreated or treated with Axl or MerTK inhibitors. However, Axl inhibitortreated mice exhibited increased numbers of neutrophils in the lungs compared to the control and MerTK-inhibitor-treated groups. Therefore, these results suggest that signaling through the Axl receptor plays a protective role during TB, by suppressing excess lung inflammation, which is detrimental to the host during Mtb infection. (AU)

• FAPESP's process: 20/10356-0 - Characterization of the role of Axl and MerTK receptors during the experimental infection with Mycobacterium tuberculosis
• Grantee: André Aparecido dos Santos Correa
• Support Opportunities: Scholarships in Brazil - Master