Metallothioneins (MTs) are a family of metal-binding proteins virtually expressed in all organisms including prokaryotes, lower eukaryotes, invertebrates and mammals. These proteins regulate homeostasis of zinc (Zn) and copper (Cu), mitigate heavy metal poisoning, and alleviate superoxide stress. In recent years, MTs have emerged as an important, yet largely underappreciated, component of the immune system. Innate and adaptive immune cells regulate MTs in response to stress stimuli, cytokine signals and microbial challenge. Modulation of MTs in these cells in turn regulates metal ion release, transport and distribution, cellular redox status, enzyme function and cell signaling. While it is well established that the host strictly regulates availability of metal ions during microbial pathogenesis, we are only recently beginning to unravel the interplay between metal-regulatory pathways and immunological defenses. In this perspective, investigation of mechanisms that leverage the potential of MTs to orchestrate inflammatory responses and antimicrobial defenses has gained momentum. The purpose of this review, therefore, is to illumine the role of MTs in immune regulation. We discuss the mechanisms of MT induction and signaling in immune cells and explore the therapeutic potential of the MT-Zn axis in bolstering immune defenses against pathogens.

Only a handful of the more than 100,000 fungal species on our planet cause disease in humans, yet the number of life-threatening fungal infections in patients has recently skyrocketed as a result of advances in medical care that often suppress immunity intensely. This emerging crisis has created pressing needs to clarify immune defense mechanisms against fungi, with the ultimate goal of therapeutic applications. Herein, we describe recent insights in understanding the mammalian immune defenses deployed against pathogenic fungi. The review focuses on adaptive immune responses to the major medically important fungi and emphasizes how dendritic cells and subsets in various anatomic compartments respond to fungi, recognize their molecular patterns, and signal responses that nurture and shape the differentiation of T cell subsets and B cells. Also emphasized is how the latter deploy effector and regulatory mechanisms that eliminate these nasty invaders while also constraining collateral damage to vital tissue.

Fungal diseases have been dramatically increasing over the past several decades. Antifungal drugs have been the mainstay of prevention and treatment of the mycoses, but in this review the authors provide strong rationale for vaccines as a preventive measure. The article considers an array of carbohydrate and protein fungal antigens against which immune responses are protective in animal models of human disease. Protection can occur as a result of vaccine formulations comprising single or multiple purified or synthetic antigens, or as a result of whole-cell vaccines. Support is provided for pursuing vaccines that induce both antibody and cell-mediated immune responses against the candidate antigens. The mechanisms of vaccine-mediated protection vary, depending in large part on the nature of the fungal agent in question. The authors discuss the fact that antibodies against carbohydrate antigens, as in the case of host defence against candidiasis, protect by opsonization or by promoting rapid ingestion of the fungal cells by host phagocytes. Antibodies against carbohydrates from the causative agent of cryptococcosis function by modulating host inflammatory responses and enhancing cell-mediated immunity. In response to protein antigens, a subset of CD4⁺ T cells and T cell cytokines, such as interferon γ and IL-12, are discussed as being central to host defence against fungi, most notably those that cause histoplasmosis, blastomycosis and coccidioidomycosis. Indeed, an absence of key T-cell subsets can compromise the vigor of an immunological response to a given vaccine. For optimal anti-fungal immunity, the authors suggest that it might be desirable to stimulate broader immune responses than can be elicited simply with a single protein antigen or clone of T cells. This could be done by using a complex immunogen that contains multiple antigens, an antigen together with a new and more potent adjuvant, or even a genetically engineered live attenuated fungus. Alternatively, new formulations or delivery methods of vaccines, including in the form of nucleic acids, might prove fruitful for this purpose. Recent findings indicate that immunity to fungi might include elements of the immune system not traditionally considered to be important, especially under circumstances in which the immune system is impaired. Therefore, the loss of select cytokines or subsets of T cells can sometimes be compensated by other cytokines or T cells. This finding raises the possibility that vaccines can be developed that recruit residual elements of immunity in compromised hosts who are otherwise at high risk of fungal infections. The review concludes with considering the possibility of the development of universal vaccine approaches. This idea was promoted by reports on cross-generic protection induced by a β-glucan vaccine, but the authors suggest that a universal vaccine would be one that advantages both antibody and cell-mediated immune responses.

Aging results in profound immune dysfunction, resulting in the decline of vaccine responsiveness previously attributed to irreversible defects in the immune system. In addition to increased interleukin-6 (IL-6), we found aged mice exhibit increased systemic IL-10 that requires forkhead box P3–negative (FoxP3⁻), but not FoxP3⁺, CD4⁺T cells. Most IL-10–producing cells manifested a T follicular helper (Tfh) phenotype and required the Tfh cytokines IL-6 and IL-21 for their accrual, so we refer to them as Tfh10 cells. IL-21 was also required to maintain normal serum levels of IL-6 and IL-10. Notably, antigen-specific Tfh10 cells arose after immunization of aged mice, and neutralization of IL-10 receptor signaling significantly restored Tfh-dependent antibody responses, whereas depletion of FoxP3⁺ regulatory and follicular regulatory cells did not. Thus, these data demonstrate that immune suppression with age is reversible and implicate Tfh10 cells as an intriguing link between “inflammaging” and impaired immune responses with age.

The presence of functional T cells is often required for successful resolution of infections with intracellular pathogens, yet the mechanisms by which they contribute to elimination of the invading pathogen in primary and secondary immunity are only partly understood. We report that increased mortality of naive α/β TCR+ or CD4+ T cell-depleted mice infected with the fungus Histoplasma capsulatum is associated with impairment of IFN-γ production. Upon secondary infection, mice concomitantly depleted of CD4+ and CD8+ cells exhibit decreased survival beyond day 25 of rechallenge, whereas elimination of either T cell subset or B cell deficiency does not result in accelerated mortality compared with controls. Remarkably, despite a decrease of H. capsulatum CFU in lungs of CD4+ plus CD8+-deficient mice, a progressive increase in spleen CFU is observed. The ability to control fungus growth in lungs is associated with vigorous TNF-α, but not IFN-γ, production by bronchoalveolar lavage cells. In contrast, spleen cells from CD4+ plus CD8+-deficient mice are unable to produce TNF-α. Thus, the cellular and molecular requirements for protective immunity vary between primary and secondary infection. Furthermore, in secondary histoplasmosis, a sharp contrast can be drawn between lungs and spleens in their reliance upon T cells to control fungal replication. The opposing activities of these organs can be ascribed in part to differential production of TNF-α.