Advances in combating fungal diseases: vaccines on the threshold

Abstract

The dramatic increase in fungal diseases in recent years can be attributed to the increased aggressiveness of medical therapy and other human activities. Immunosuppressed patients are at risk of contracting fungal diseases in healthcare settings and from natural environments. Increased prescribing of antifungals has led to the emergence of resistant fungi, resulting in treatment challenges. These concerns, together with the elucidation of the mechanisms of protective immunity against fungal diseases, have renewed interest in the development of vaccines against the mycoses. Most research has used murine models of human disease and, as we review in this article, the knowledge gained from these studies has advanced to the point where the development of vaccines targeting human fungal pathogens is now a realistic and achievable goal.

Figure 1. The host response to fungi

The figure shows the complex interaction between fungi or fungal antigens and the host immune response. Dendritic cells (DCs) process and present antigens through class I or class II major histocompatibility complex (MHC) molecules to antigen-specific clones of T cells endowed with the capacity to recognize the peptide epitopes through specific T-cell receptors (TCR). The production of interleukin (IL)-12 by DCs leads to the outgrowth of T-helper 1 (T_H1) cells that produce interferon-γ (IFN-γ), tumour necrosis factor-α (TNF-α) or both. The T_H1 response leads to enhanced fungistatic and fungicidal activities by phagocytes. The induction of IL-4 (and failure to produce IL-12) by DCs leads to a T_H2 response, which blunts the generation of protective immunity. PRR, protein recognition receptor.

Figure 2. Antibody-mediated protection against fungal disease

The mechanisms described for antibody-mediated defence against bacterial agents are presumed or proven to also be operative against fungi, including direct antibody (Ab) neutralization of fungal toxins and extracellular enzymes, and direct inhibition of fungal growth. Antibodies can indirectly inhibit fungi by functioning as an opsonin, either alone or in conjunction with complement factor C3, which is activated and deposited as C3b and which degrades to iC3b on the fungal surface. Antibody and complement-coated fungal cells interact with Fc receptors (FcR) and complement receptors (CR) on host phagocytic cell membranes, resulting in prompt ingestion of the fungal cell and which can lead to the death of the ingested fungal cell. In defence against intracellular fungal pathogens, such as Cryptococcus neoformans, protective antibodies seem to have a role in modulating host inflammatory responses and enhancing the organization of T-cell responses.

Figure 3. The crucial role of the T-helper 1 (T_H1) response in vaccine-induced immunity to fungi

In experimental models, recombinant heat shock protein 60 (rHsp60) induces resistance in immunocompetent animals to lethal challenge with Histoplasma capsulatum (left). CD4⁺ T cells and the T_H1 cytokines interleukin (IL)-12, IL-10 and interferon γ (IFN-γ) are required for this resistance. In immunocompetent animals, the resistance that is induced against experimental blastomycosis by a live attenuated strain of Blastomyces dermatitidis (with a mutation in the Blastomyces adhesin 1 (bad1) gene) has similar requirements for CD4⁺ T cells and T_H1 cytokines (right). However, the immune system shows some plasticity and can compensate for the absence of particular cells or cytokines if they are absent during the induction or afferent phase of the vaccine-induced immune response (rather than being eliminated during the expression or efferent phase of the response). For example, immunodeficient animals that lack either CD4⁺ T cells or selected T_H1 cytokines at the time of vaccination could control the live attenuated vaccine following subcutaneous administration, and use other T cells or T_H1 cytokines to engender vaccine resistance to lethal experimental disease. The absence of IFN-γ is compensated by tumour necrosis factor α (TNF-α) and granulocyte–macrophage colony-stimulating factor (GM–CSF), and the loss of CD4⁺ T cells is compensated by CD8⁺ T cells that produce type 1 cytokines. Δ, gene knockout or missing owing to antibody depletion.

Figure 4. Antigen processing and presentation to T cells by major histocompatibility complex (MHC) class I and class II molecules

a | In the endogenous processing pathway, proteins produced in the cytosol of phagocytic or non-phagocytic cells are cleaved in the proteasome into short peptide fragments of 8–10 amino acids and transported into the endoplasmic reticulum (ER) through TAP1 and TAP2, where they bind to MHC class I molecules. The peptide–MHC class I complex is transported through the Golgi apparatus to the cell surface where it is recognized by CD8⁺ T cells. b | In the exogenous processing pathway, antigen-presenting cells such as macrophages or dendritic cells take up extracellular proteins and other microbial products by endocytosis or phagocytosis. In acidified endosomes, MHC class II molecules that have been transported from the Golgi bind fragmented antigenic peptides that are 10–25 amino acids in length. Peptide–MHC class II complexes are displayed on the cell surface for recognition by CD4⁺ T cells. c | During cross-presentation, dendritic cells take up particles or even phagocytes that have internalized microorganisms. Through an ill-defined process, proteolytically cleaved fragments of antigenic peptide from the phagosome ‘cross over’ and enter into the MHC class I pathway, probably binding molecules from the ER. These peptide–MHC class I molecules move to the cell surface where they can be recognized by CD8⁺ T cells. TCR, T-cell receptor.

Figure 5. The profiles of the anti-fungal immune responses induced by various immunogens

a | In DNA vaccination, the plasmid DNA contains a fungal gene that is often under the control of an active promoter. Other genes can be co-expressed, for example cytokines such as interleukin (IL)-12 that can exert adjuvant effects. The fungal proteins are translated and from the cytosol enter the endogenous route of antigen processing, as shown in Fig. 4a. Therefore, this vaccine strategy primes CD8 ⁺ T cells, which function by producing T-helper 1 (T_H1) cytokines, and perhaps lytic products such as perforin or granulysin. A secretion signal (not shown) can be engineered onto the antigen sequence so that the expressed gene product is exported out of the cell, and thereby enters the exogenous antigen-processing pathway, as shown in Fig. 4b, priming CD4⁺ T cells and their soluble products. b | In vaccination with a protein antigen, the soluble protein and an adjuvant enter the exogenous antigen-processing pathway, which chiefly primes CD4⁺ T cells. These cells perform helper and regulatory functions by releasing various soluble products, which arm phagocytes, balance the unrestrained vigor of the immune response, and help B cells produce antibody. c | A live attenuated vaccine can enter antigen-presenting cells, especially dendritic cells, by multiple routes. The vaccine primes CD4⁺ T cells through the exogenous antigen-processing pathway pathway and CD8⁺ T cells through cross presentation. This engages multiple arms of the immune response, including multiple T-cell subsets and their accompanying products, and B cells and antibody production. GM-CSF, granulocyte–macrophage colony-stimulating factor; IFN, interferon; MHC, major histocompatibility complex; TCR, T-cell receptor; TGF, transforming growth factor; TNF, tumour necrosis factor; T-reg, regulatory T cell.