Role of prrs (tlr2 and dectin-1) in hematopoietic stem and progenitor cell differentiationimplications in protection against candida albicans infection

Resumen

Detection of infection by hematopoietic stem and progenitor cells (HSPCs) is essential to replace myeloid cells consumed during the immune response. HSPCs express some functional pattern recognition receptors involved in the recognition of Candida albicans. In this context, our group has previously demonstrated that C. albicans yeasts induce proliferation and differentiation of HSPCs via TLR2 and Dectin-1. In the present PhD thesis, we used in vitro and ex vivo models of HSPC differentiation to investigate the functional consequences for mature myeloid cells of exposure of HSPCs to PAMPs or C. albicans yeasts. In vitro experiments show that murine HSPCs continuously exposed to TLR2 or TLR4 ligands, in homeostatic conditions, generate macrophages with a diminished ability to secrete TNF-α and IL-6 (tolerized phenotype), whereas Dectin-1 or C. albicans yeasts stimulation leads to the generation of macrophages that secrete higher amounts of these pro-inflammatory cytokines (trained phenotype). In these conditions, transient exposure of HSPCs to TLR agonists is sufficient to generate macrophages with a tolerized phenotype, which is partially reversed by subsequent exposure to C. albicans yeasts. However, only TLR2 signaling in HSPCs impacts the inflammatory function of macrophages they produce in inflammatory conditions, and subsequently exposure to C. albicans yeasts do not reverse but reinforce this altered phenotype. TLR2 and Dectin-1 signaling in HSPCs also have functional consequences for the antigen presenting cells (APCs) derived from them, which exhibit an altered expression of histocompatibility complex class II molecules (signal one) and costimulatory molecules (CD40, CD80 and CD86) (signal two), as well as an altered cytokine production (TNF-α, IL-6, IL-12 p40 and IL-2) (signal three). These changes in the three key signals that APCs deliver to naïve T cells provoke that APCs from TLR2/Dectin-1-programed HSPCs prime enhanced Th1 and Th17 responses in CD4⁺ T cell cocultures. C. albicans systemic infection and in vivo TLR2 agonist treatment also alter cytokine production and fungicidal activity of the ex vivo macrophages derived from murine HSPCs in homeostatic conditions. Unexpectedly, an extended TLR2 agonist treatment generates macrophages with a trained phenotype. This prolonged treatment also causes an expansion of HSPCs and myeloid cells in the spleen, and drastically reduces the fungal burden in the kidney and the spleen during systemic C. albicans infection. This protection is abrogated by immunodepletion of HSPCs. In addition, HSPCs produce in vitro cytokines and chemokines in response to a TLR2 ligand or C. albicans yeasts, and these secretomes can induce differentiation of HSPCs towards the myeloid lineage. Taken together, these data assign an active role to HSPCs in sensing pathogens during infection and in contributing to host protection by diverse mechanisms.