TLR3 essentially promotes protective class I–restricted memory CD8+ T-cell responses to Aspergillus fumigatus in hematopoietic transplanted patients

Abstract

Aspergillus fumigatus is a model fungal pathogen and a common cause of severe infections and diseases. CD8 ؉ T cells are present in the human and murine T-cell repertoire to the fungus. However, CD8 ؉ T-cell function in infection and the molecular mechanisms that control their priming and differentiation into effector and memory cells in vivo remain elusive. In the present study, we report that both CD4 ؉ and CD8 ؉ T cells mediate protective memory responses to the fungus contingent on the nature of the fungal vaccine. Mechanistically, class I MHCrestricted, CD8 ؉ memory T cells were activated through TLR3 sensing of fungal RNA by cross-presenting dendritic cells. Genetic deficiency of TLR3 was associated with susceptibility to aspergillosis and concomitant failure to activate memory-protective CD8 ؉ T cells both in mice and in patients receiving stem-cell transplantations. Therefore, TLR3 essentially promotes antifungal memory CD8 ؉ T-cell responses and its deficiency is a novel susceptibility factor for aspergillosis in high-risk patients. IntroductionTLR3 plays a key role in modulating inflammation and innate immunity in the airway. Although best known for recognition of viral double-stranded RNA (dsRNA) and its synthetic analog polyinosinic:polycytidylic acid [poly(I:C)], 1 TLR3 also recognizes endogenous ligands, 2 including heterologous RNA released from or associated with necrotic cells or generated by in vitro transcription. 3 Therefore, TLR3, together with other intracellular signaling proteins, 4 induces or otherwise modulates innate immune responses and inflammation in settings that are not associated with viral dsRNA. TLR3 signaling may also modulate adaptive immune responses by providing cross-priming of cytotoxic T lymphocytes through signaling in dendritic cells (DCs) 5,6 via type I IFNs 7 and in the absence of CD4 ϩ T-cell help. 8 Therefore, it is not surprising that people with mutations in key TLR3 signaling components have a selective immunodeficiency manifested by recurrent episodes of herpes simplex virus 1 encephalitis 9,10 or enteroviral myocarditis/cardiomyopathy. 11 Aspergillus fumigatus is a model fungal pathogen and a common cause of severe infections and diseases. Humans inhale hundreds of conidia every day without adverse consequences, 12 except for a minority of persons in whom defense systems fail and a life-threatening form of disease can develop. CD4 ϩ and CD8 ϩ T cells are present in the human T-cell repertoire to the fungus 13-15 and adoptive transfer of A fumigatus-specific CD4 ϩ T cells conferred protection against invasive fungal infection. 15,16 Recent studies indicate a role for TLR3 in murine aspergillosis. By functioning as an endogenous sensor of fungal RNA, 17 TLR3 mediates expression of the enzyme indoleamine 2,3-dioxygenase (IDO) on both epithelial cells 18 and DCs,19 contributing to the local regulation of innate and adaptive inflammation to the fungus. However, the findings that protective memory CD8 ϩ T cells are induced against fungi 20-23 suggest a possible rol...

CD4+ T cell vaccination overcomes defective cross-presentation of fungal antigens in a mouse model of chronic granulomatous disease

“…Murine studies have shown that both CD8 + and CD4 + T cells mediate memory responses to fungi via the production of effector cytokines, such as IFN-γ, and, in the case of CD8 + T cells, through cytotoxic activity against fungus-laden cells (7)(8)(9)(10)(11). The requirement for IFN-γ in vaccine-mediated resistance was confirmed by vaccination of Ifng -/-or Il17ra -/-mice with Aspergillus conidia or Pep1p.…”

“…Despite the lack of evidence for susceptibility to aspergillosis in patients with congenital absence of T cells (2), CD4 + and CD8 + T cells are present in the human T cell repertoire to the fungus (3)(4)(5), and adoptive transfer of A. fumigatus-specific CD4 + T cells conferred protection against invasive aspergillosis (5,6). As with other fungi (7-10), antigen-specific CD4 + and CD8 + T cell memory to A. fumigatus has been demonstrated in experimental aspergillosis (11). However, the molecular mechanisms leading to T cell memory are largely unknown.…”

Immune reconstitution post allogeneic transplant and the impact of immune recovery on the risk of infection

“…96 After BM transplantation, T cell proliferative responses to specific antigens, such as candidin, tetanus toxoid, tuberculin or toxoplasma, are absent in about 20-50% of patients, even up to a year after transplantation. 20 Primary T cell immunity against fungi is provided by CD4C Th1 and Th17 cells [158][159][160][161] as well as memory CD8C T-cell cells. 160 Consequently, at 3 months or later after HSCT, fewer than a quarter of patients will have detectable functional T cell responses against the most common invasive fungal infection.…”

NADPH oxidase regulates efficacy of vaccination in aspergillosis

“…Less well appreciated is the influence of CD8 + T cells on the host response. Antigen-reactive CD8 + T cells can be elicited by Aspergillus antigens, and protection relies on recognition of fungal RNA by TLR3 (11,12). That T cells exert a strong influence on the host response has major implications not only for the immunopathogenesis but also for the design of prophylactic or therapeutic vaccines for this pathogen.…”

“…While blockade of PD-1 enhances HIV- and SIV-specific T cell cytokine production and proliferation in vitro (9, 10), initially there was skepticism that this method would restore immunological function in the pathogenic RM SIV model in vivo (12). This skepticism was based on the premise that if in vivo blockade of the PD-1 inhibitory receptor in chronic SIV infection could result in enhanced antiviral T and B cell responses, it could also potentially exacerbate bystander T cell activation in an already persistent inflammatory setting, potentially leading to accelerated disease progression.…”