Regulatory T Cells Enhance Susceptibility to Experimental Trypanosoma Congolense Infection Independent of Mouse Genetic Background

Abstract

BackgroundBALB/c mice are highly susceptible while C57BL/6 are relatively resistant to experimental Trypanosoma congolense infection. Although regulatory T cells (Tregs) have been shown to regulate the pathogenesis of experimental T. congolense infection, their exact role remains controversial. We wished to determine whether Tregs contribute to distinct phenotypic outcomes in BALB/c and C57BL/6 mice and if so how they operate with respect to control of parasitemia and production of disease-exacerbating proinflammatory cytokines.Methodology/FindingsBALB/c and C57BL/6 mice were infected intraperitoneally (i.p) with 103 T. congolense clone TC13 and both the kinetics of Tregs expansion and intracellular cytokine profiles in the spleens and livers were monitored directly ex vivo by flow cytometry. In some experiments, mice were injected with anti-CD25 mAb prior or post T. congolense infection or adoptively (by intravenous route) given highly enriched naïve CD25+ T lymphocytes prior to T. congolense infection and the inflammatory cytokine/chemokine levels and survival were monitored. In contrast to a transient and non significant increase in the percentages and absolute numbers of CD4+CD25+Foxp3+ T cells (Tregs) in C57BL/6 mouse spleens and livers, a significant increase in the percentage and absolute numbers of Tregs was observed in spleens of infected BALB/c mice. Ablation or increasing the number of CD25+ cells in the relatively resistant C57BL/6 mice by anti-CD25 mAb treatment or by adoptive transfer of CD25+ T cells, respectively, ameliorates or exacerbates parasitemia and production of proinflammatory cytokines.ConclusionCollectively, our results show that regulatory T cells contribute to susceptibility in experimental murine trypanosomiasis in both the highly susceptible BALB/c and relatively resistant C57BL/6 mice.

Low-Dose Intradermal Infection with Trypanosoma congolense Leads to Expansion of Regulatory T Cells and Enhanced Susceptibility to Reinfection

“…In addition, infected mice also show a reduced antibody response to sheep red blood cells (SRBC) following immunization (44)(45)(46)(47)(48). Several mechanisms have been proposed as the cause of trypanosomeinduced immunosuppression, including macrophages (49), suppressor T cells (50), nitric oxide (NO) (51), and Tregs (24,25). Recent reports (including those from our laboratory) have shown that conventional and naturally occurring Tregs (characterized as CD4 ϩ CD25 ϩ Foxp3 ϩ ) contribute to enhanced susceptibility to experimental T. congolense infection in mice (24,25).…”

“…In experiments requiring Treg depletion, each mouse received 100 g of anti-CD25 monoclonal antibody (MAb) (clone PC61) intraperitoneally. Previous studies from our lab have shown that this dose of antibody causes sustained depletion of CD25 ϩ Foxp3 ϩ T cells (without affecting other cell populations) for up to 7 days in mice infected with T. congolense (25,30). In vivo depletion of macrophages was done by intravenous injection of 200 l of chlodronateloaded liposomes (5 mg/ml; Encapsular NanoSciences LLC, Nashville, TN) 24 h before primary intradermal infection.…”

“…Because we and others have previously shown that regulatory T cells (Tregs) prevent the control of infection with T. congolense (24,25), we wondered whether lowdose intradermal infection was associated with expansion of these cells. Therefore, we infected mice intradermally with 10 3 T. congolense parasites and on the indicated days assessed the percentages and absolute numbers of CD4 ϩ CD25 ϩ Foxp3 ϩ regulatory T cells in the spleens and draining lymph nodes by flow cytometry.…”

“…Because previous studies showed that Tregs negatively influence the outcome of T. congolense infection in mice (24,25), we hypothesized that depletion of Tregs during challenge infection will abrogate the enhanced susceptibility mediated by repeated low-dose intradermal infection. We therefore injected mice repeatedly with 10 2 T. congolense parasites (2 times with a 1-week interval), and on the third week injected them with anti-CD25 monoclonal antibody (PC61) to deplete Tregs as we have done previously (25,30). Twenty-four hours later, mice were challenged intradermally with 10 3 T. congolense parasites.…”

“…Although their primary role is to prevent autoimmunity and suppress inflammatory responses, Tregs have also been implicated in the pathogenesis of several infectious diseases, including those caused by parasites (22)(23)(24). In particular, increased numbers of CD4 ϩ CD25 ϩ Foxp3 ϩ Tregs have been reported in experimental T. congolense infections (25,26), and these cells have been implicated in enhanced susceptibility to the infection (24,25), although the exact mechanisms remain unknown.…”