Interferon-γ Possesses Anti-Microbial and Immunomodulatory Activity on a Chlamydia trachomatis Infection Model of Primary Human Synovial Fibroblasts

Abstract

Chlamydia trachomatis, an obligate intracellular pathogen, is the most common cause of bacterial sexually transmitted diseases, and it is potentially responsible for severe chronic sequelae, such as reactive arthritis. To date, details of the mechanisms by which Chlamydiae induce innate antimicrobial pathways in synovial fibroblasts, are not well characterized; therefore, herein, we investigated the effects of interferon (IFN)α, IFNβ, and IFNγ on the infection, and replication phases of the C. trachomatis developmental cycle, as well as on the induction of pattern recognition receptors (PRRs) and IFN-related pathways. To do so, we set up an in vitro chlamydial-infection model of primary human synovial cells treated with IFNs before or after the infection. We then determined the number of chlamydial inclusion forming units and inclusion size, as well as the expression of toll like receptor (TLR)2, TLR3, TLR4, cyclic GMP-AMP synthase (cGAS), stimulator of IFN gene (STING), IRF9, ISG56, and GBP1. The main result of our study is the significant inhibition of C. trachomatis infection and replication in human synovial cells following the treatment with IFNγ, whereas IFN-I proved to be ineffective. Furthermore, IFNγ greatly upregulated all the PRRs and ISGs examined. In conclusion, IFNγ exhibited a potent anti-Chlamydia activity in human synovial cells as well as the ability to induce a strong increase of innate immune pathways.

Potential IFNγ Modulation of Inflammasome Pathway in Chlamydia trachomatis Infected Synovial Cells

“…Although the basis for caspase-1 stimulation of chlamydial infection is not well defined [32] and remains poorly characterized in our human synovial cell model, we found that exposure of C. trachomatis-infected cells to IFN-γ may modulate the canonical inflammasome pathway by reducing the gene expression of caspase-1. These findings support our previous work on anti C. trachomatis activity of IFN-γ and yield novel insights into mechanisms by which this cytokine could interfere with its replication [11]. Previous studies have shown that caspase-1 can be upregulated by IFN-γ in distinct cell lines [33][34][35].…”

“…Caspase-1 activated IL-18 is also a potent inducer of IL-1β, IL-8, and interferon (IFN)γ [20,21], which has been demonstrated to play a key role in the clearance and protection against C. trachomatis by modulating a plethora of host cell signaling pathways [22,23]. Interestingly, in our recent study, the anti-chlamydial activity of IFN-γ has also been demonstrated in primary human synovial fibroblasts through the modulation of the hostcell innate immune responses, as evidenced by an increased expression of TLR2, 3 and 4, and IFN-related pathways, including GAS, STING, IRF9, ISG56, and GBP1 [11].…”

“…Human synovial cells were grown on 24-well cell-culture plates or on 25 cm 2 cellculture flasks. Upon confluence, cell monolayers were either pre-incubated with IFN-γ at a concentration of 10 3 U/mL in DMEM with 15% FBS, or pre-incubated in DMEM with 15% FBS alone, at 37 • C in humidified atmosphere with 5% CO 2 , as previously described [11]. After 24 h, cell monolayers were washed with PBS, infected with C. trachomatis at a multiplicity of infection (MOI) of 1.0 as above described, and then incubated at 37 • C for 6, 18, 24 and 36 h.p.i.…”

“…Following chlamydial infection, the host cell response begins with the activation of a complex network of immune receptors and their unique downstream signaling pathways, resulting in the induction of proinflammatory cytokines involved in either the elimination of C. trachomatis or tissue damage related to chronic inflammatory state. In particular, Toll-like receptors (TLRs), TLR2, TLR3 and TLR4, and oligomerization domain (NOD)-like receptors (NLRs), NOD1, as well as the respective downstream signaling pathways, like NFκb, IRF9 and MyD88, have been identified in the immune recognition of C. trachomatis in several in vivo and in vitro models [5,[7][8][9][10][11].…”

“…The ability of C. trachomatis to infect synovial cells, as evidenced by the presence of chlamydial inclusions in human synovial fibroblasts [11], suggested its potential direct role in the development of ReA. Chlamydial inclusion is unique among intracellular pathogens Life 2021, 11, 1359 2 of 14 and represents a chlamydial survival strategy since it provides a protected vacuole where C. trachomatis replicates [3,12].…”