Antifungal Tc17 cells are durable and stable, persisting as long-lasting vaccine memory without plasticity towards IFNγ cells

Our understanding of persistence and plasticity of IL-17A⁺ memory T cells is clouded by conflicting results in models analyzing T helper 17 cells. We studied memory IL-17A⁺ CD8⁺ T-cell (Tc17) homeostasis, persistence and plasticity during fungal vaccine immunity. We report that vaccine-induced memory Tc17 cells persist with high fidelity to the type 17 phenotype. Tc17 cells persisted durably for a year as functional IL-17A⁺ memory cells without converting to IFNγ⁺ (Tc1) cells, although they produced multiple type I cytokines in the absence of residual vaccine antigen. Memory Tc17 cells were canonical CD8⁺ T cells with phenotypic features distinct from Tc1 cells, and were Ror(γ)t^hi, TCF-1^hi, T-bet^lo and EOMES^lo. In investigating the bases of Tc17 persistence, we observed that memory Tc17 cells had much higher levels of basal homeostatic proliferation than did Tc1 cells. Conversely, memory Tc17 cells displayed lower levels of anti-apoptotic molecules Bcl-2 and Bcl-xL than Tc1 cells, yet were resistant to apoptosis. Tc1 cells required Bcl-2 for their survival, but Bcl-2 was dispensable for the maintenance of Tc17 cells. Tc17 and Tc1 cells displayed different requirements for HIF-1α during effector differentiation and sustenance and memory persistence. Thus, antifungal vaccination induces durable and stable memory Tc17 cells with distinct requirements for long-term persistence that distinguish them from memory Tc1 cells.

CD4⁺ T-cell deficient patients such as those with AIDS and idiopathic CD4⁺ T-cell lymphopenia are vulnerable to systemic fungal infections. We previously showed that CD8⁺ T cells can be exploited in CD4⁺ T cell deficient hosts for vaccine immunity against lethal fungal pneumonia in mice and that IL-17A production by these cells (Tc17) is essential. Existing dogma holds that IL-17A producing CD4⁺ T cells (Th17) are highly plastic, unstable, and convert into IFNγ producing cells, losing the capacity to produce IL-17A, which is the signature feature of Tc17 cells. Here, we show that vaccine-elicited antifungal Tc17 cells are maintained as stable and long-lasting memory cells that resist conversion into IFNγ cells (Tc1) and protect CD4⁺ T cell deficient hosts against lethal pulmonary fungal infection. Antifungal Tc17 cells displayed features that define classical memory cells. However, memory Tc17 exhibited different requirements than Tc1 cells in the factors that promote T cell survival, including anti-apoptotic molecules Bcl-2 and Bcl-xl, and HIF-1α, which aids survival of cells in lower oxygen conditions found during inflammation. Thus, our study reveals that fungal vaccination elicits a durable, stable population of Tc17 cells with distinct features of survival needed for preventing infection in immunocompromised hosts.

V beta1+ J beta1.1+/V alpha2+ J alpha49+ CD4+ T cells mediate resistance against infection with Blastomyces dermatitidis

Immunization with a cell wall/membrane (CW/M) and yeast cytosol extract (YCE) crude antigen from Blastomyces dermatitidis confers T-cell-mediated resistance against lethal experimental infection in mice. We isolated and characterized T cells that recognize components of these protective antigens and mediate protection. CD4+ T-cell clones elicited with CW/M antigen adoptively transferred protective immunity when they expressed a V alpha2+ J alpha49+/V beta1+ J beta1.1+ heterodimeric T-cell receptor (TCR) and produced high levels of gamma interferon (IFN-gamma). In contrast, V beta8.1/8.2+ CD4+ T-cell clones that were reactive against CW/M and YCE antigens and produced little or no IFN-gamma either failed to mediate protection or exacerbated the infection depending on the level of interleukin-5 expression. Thus, the outgrowth of protective T-cell clones against immunodominant antigens of B. dermatitidis is biased by a combination of the TCR repertoire and Th1 cytokine production.

IL-12 is required for induction but not maintenance of protective, memory responses to Blastomyces dermatitidis: implications for vaccine development in immune-deficient hosts

Cellular immunity mediated by T lymphocytes, in particular CD4(+) and CD8(+) type 1 (T1) cells, is the main defense against pathogenic fungi. IL-12 initiates T1 cell development and cell-mediated immunity, but it is unclear whether IL-12 contributes to the maintenance of an antifungal T1 response. In this study, we addressed the role of IL-12 for vaccine-induced memory T cell development against experimental pulmonary blastomycosis. CD4(+) T cells absolutely required IL-12 to control a live genetically engineered attenuated strain of Blastomyces dermatitidis given s.c. as a vaccine, whereas CD8(+) T cells were significantly less dependent on IL-12. Despite differential dependency of T cell subsets on IL-12 during vaccination, neither subset acquired memory immunity in the absence of IL-12. In contrast, adoptive transfer of immune CD4 T cells from wild-type mice into IL-12(-/-) mice showed that CD4(+) T1 memory cells sustained a T1 cytokine profile and remained protective over a period of 6 mo posttransfer. Similarly, memory CD8 cells elicited in IL-12(-/-) mice with killed yeast and transient rIL-12 treatment (during vaccination) remained durable and protective after animals were rested for 3 mo. In conclusion, these studies demonstrate that once CD4 and CD8 cells have acquired a protective T1 phenotype they no longer require the presence of IL-12 to maintain antifungal protective memory.

Progress in vaccination for histoplasmosis and blastomycosis: coping with cellular immunity

Human infection with Histoplasma capsulatum or Blastomyces dermatitidis is sufficiently frequent to warrant exploring the development of vaccines. This review examines the advancements that have been accomplished over the last few years. The availability of molecular tools to create recombinant antigens or mutant strains has produced a small number of useful vaccine candidates. More importantly, the studies summarized herein demonstrate that understanding the host response to a protein or mutant fungus is critical to creating a vaccine that may be useful for the immunocompromised patient.

Vaccine immunity to pathogenic fungi overcomes the requirement for CD4 help in exogenous antigen presentation to CD8+ T cells: implications for vaccine development in immune-deficient hosts

Systemic fungal infections with primary and opportunistic pathogens have become increasingly common and represent a growing health menace in patients with AIDS and other immune deficiencies. T lymphocyte immunity, in particular the CD4+ Th 1 cells, is considered the main defense against these pathogens, and their absence is associated with increased susceptibility. It would seem illogical then to propose vaccinating these vulnerable patients against fungal infections. We report here that CD4+ T cells are dispensable for vaccine-induced resistance against experimental fungal pulmonary infections with two agents, Blastomyces dermatitidis an extracellular pathogen, and Histoplasma capsulatum a facultative intracellular pathogen. In the absence of T helper cells, exogenous fungal antigens activated memory CD8+ cells in a major histocompatibility complex class I-restricted manner and CD8+ T cell-derived cytokines tumor necrosis factor alpha, interferon gamma, and granulocyte/macrophage colony-stimulating factor-mediated durable vaccine immunity. CD8+ T cells could also rely on alternate mechanisms for robust vaccine immunity, in the absence of some of these factors. Our results demonstrate an unexpected plasticity of immunity in compromised hosts at both the cellular and molecular level and point to the feasibility of developing vaccines against invasive fungal infections in patients with severe immune deficiencies, including those with few or no CD4+ T cells.

Serological differences in three Blastomyces dermatitidis strains

Yeast phase lysate antigens, prepared from three isolates of Blastomyces dermatitidis (T-58, Tennessee dog; 48089, Zaire human; ERC-2, Wisconsin dog) were assayed for their ability to detect antibodies in human sera, dog sera and sera from rabbits immunized with each of the lysate antigens. The dog sera were from animals diagnosed with blastomycosis from various endemic regions in North America. T-58 and ERC-2 lysate antigens exhibited a high reactivity with the serum from dogs infected with blastomycosis; however, 48089 lysate showed low reactivity with the same sera. With the immunized rabbit sera, 48089 lysate was the only lysate with a high reactivity with the 48089 serum and it exhibited little reactivity with the heterologous sera. The T-58 and ERC-2 lysate antigens reacted minimally with the 48089 serum but reacted highly with both the T-58 and ERC-2 sera. The human sera were from individuals potentially exposed to B. dermatitidis while working on a prairie dog relocation project in Colorado. Remarkably, all three lysate antigens could detect antibodies in the individuals diagnosed with blastomycosis. This study indicated that there were serological differences in the 48089 Zaire lysate compared with the other lysate antigens and it may be designated serotype 2.

Mutation of the WI-1 gene yields an attenuated blastomyces dermatitidis strain that induces host resistance

Systemic fungal infections are becoming more common and difficult to treat, and vaccine prevention is not available. Pulmonary infection with the dimorphic fungus Blastomyces dermatitidis often progresses and requires treatment to prevent fatality. We recently created a recombinant strain of the fungus lacking the WI-1 adhesin and pathogenicity. We show here that administration of viable yeast of this attenuated strain vaccinates against lethal pulmonary experimental infection due to isogenic and nonisogenic strains from diverse geographic regions. To our knowledge, this is the first example of a recombinant attenuated vaccine against fungi. The vaccine induces delayed-type hypersensitivity and polarized type 1 cytokine responses, which are linked with resistance. A cell-wall/membrane (CW/M) antigen from the vaccine strain also induces polarized and protective immune responses. Lymph node cells and CD4(+) T-cell lines raised with CW/M antigen transfer protective immunity when they release type 1 cytokine IFN-gamma, but not when they release IL-4, and neutralization of IFN-gamma confirmed its role in vivo. Thus, by mutating a pathogenetic locus in a dimorphic fungus, we have created an attenuated vaccine strain and have begun to elucidate fungal and host elements requisite for vaccine immunity.

Interleukin 12 as an adjuvant to WI-1 adhesin immunization augments delayed-type hypersensitivity, shifts the subclass distribution of immunoglobulin G antibodies, and enhances protective immunity to Blastomyces dermatitidis infection

Cell-mediated immunity is pivotal in host resistance to Blastomyces dermatitidis infection. Immunization of mice with the WI-1 adhesin enhances resistance against experimental pulmonary infection but elicits features of a mixed T-helper-cell immune response. Immune mice acquire delayed-type hypersensitivity (DTH) but also high titers of WI-1-specific immunoglobulin G1 (IgG1) and IgG2b, a result indicative of T-helper-2 cellular immunity. We report that interleukin-12, used as an adjuvant for WI-1 immunization, augments DTH, shifts the balance of the T-helper phenotype toward Th1, and enhances resistance to B. dermatitidis infection.

Investigation of anti-WI-1 adhesin antibody-mediated protection in experimental pulmonary blastomycosis

Infection with Blastomyces dermatitidis elicits strong antibody responses to the surface adhesin WI-1. The antibodies are directed chiefly against the adhesive domain, a 25-amino-acid repeat. Tandem-repeat-specific monoclonal antibodies (mAbs) were studied for their opsonic activity in vitro and their capacity to adoptively transfer protection in murine experimental blastomycosis. mAbs to WI-1 enhanced binding and entry of B. dermatitidis yeasts into J774. 16 cells but did not enhance killing or growth inhibition of the yeast. Passive transfer of 8 mAbs to WI-1 into 3 different inbred strains of mice also did not improve the course of experimental infection and sometimes worsened it. mu-deficient mice were more resistant to experimental blastomycosis than were intact littermates, and passive transfer of the mAbs into these mice did not protect them against experimental infection. Thus, antibody to WI-1 does not appear to improve the outcome of murine blastomycosis and may enhance the infection.

Immunogenicity and protective efficacy of the WI-1 adhesin of Blastomyces dermatitidis

People infected with Blastomyces dermatitidis develop strong immunity to the yeast surface adhesin WI-1, including antibody responses to the adhesive domain, a 25-amino-acid repeat, and cellular responses to the N terminus. We studied the immunogenicity of WI-1 and the ability of anti-WI-1 immune responses to protect against lethal pulmonary infection in mice. WI-1 immunization, given in Freund's adjuvant subcutaneously in two doses 2 weeks apart, evoked delayed hypersensitivity responses in a concentration-dependent manner. Immunized mice also had anti-WI-1 antibody responses, with titers reaching an endpoint dilution of approximately 1:800,000. Anti-WI-1 immunoglobulin G (IgG) antibody subclasses were IgG1 > IgG2b > IgG2a > IgG3, indicating a mixed T helper 1 and T helper 2 immune response. In protection experiments, WI-1 immunization significantly prolonged the survival of C57BL/6 and BALB/c mice compared to controls following intranasal administration of a lethal dose of B. dermatitidis yeasts (Kaplan-Meier survival curve P values of 0.027 to 0.0002) and also protected a proportion of the animals from death due to progressive pulmonary blastomycosis. Taken together, our results suggest that administration of WI-1 raises antibody and cell-mediated immune responses, which enhance resistance against pulmonary infection with B. dermatitidis. Mechanisms of vaccine-induced resistance require further investigation.