Current Molecular Medicine - Volume 5, Issue 4, 2005

In the last few decades, a plethora of studies have demonstrated the relevance of host acquired immunity against invasive fungal infections. The consolidated consensus that cell-mediated immunity is critical for the successful outcome of fungal infections has been hampered in recent years by the evidence of the role that antibodies may play in host immunoprotection against pathogenic fungi. Today, the major concern is neither the relative importance of cellular and humoral responses nor the modalities of functioning and cooperating of the two arms of the immune system but, rather, the mechanism of action of protective antifungal antibodies. In this issue, the latest and most comprehensive findings on the role played by antibodies in the antifungal defense are reviewed. For Candida albicans, the fungus that has generated the most extensive studies relying on experimental animal models most mimicking human vaginal candidiasis, the potential of passive immunization (i.e. immunotherapy) based on antibodies specific for very characterized virulence factors such as mannans (adhesins), aspartyl-proteinases, other enzymes, germ tube or molecules critical for viability such as heat-shock proteins or cell wall β-glucan is addressed by Antonio Cassone and collaborators. As a matter of fact, the mechanism (deposition of complement factor C3 over the entire yeast cell wall) by which anti-mannan antibodies specific for short chain β-linked oligomannosides as well as a genetically recombinant antibody against fungal heat-shock protein 90 may be protective, alone or in combination with conventional antifungal drugs, against vaginal and disseminated candidiasis is discussed in detail by Jim E. Cutler and Ruth C. Matthews with James P. Burnie respectively. According to an intriguing paradox that existing candidate chemotherapy agents which have recognized potential to increase the risk of fungus infections may represent an excellent resource for the discovery of novel antifungals and targets, the fungicidal and tumoricidal activity of a monoclonal antibody, characterized by a triple anti-C. albicans activity and directed against a proteinic epitope of a cell wall stress mannoprotein recognized to be the major target for secretory IgA, is reported by José Pontón and collaborators. More general insights into the mechanisms of antibody-mediated immunity and the dependance of the cellular response induced by the capsular polysaccharides in the presence or absence of specific protective antibodies are derived from the studies of Arturo Casadevall, Liise-anne Pirofski and Anna Vecchiarelli in Cryptococcus neoformans. The dimorphic fungus Histoplasma capsulatum is, moreover, proffered as a prototype for antibody protection against endemic mycoses by Joshua D. Nosanchuk. Furthermore, the potential of human natural, polyclonal, monoclonal and recombinant yeast killer toxin-like antibodies as well as synthetic killer mimotopes against different pathogenic fungi (including antibiotic-resistant species and strains) is proposed as a wide spectrum antifungal therapeutic approach by Luciano Polonelli's group. Overall, this review summarizes different types of antibodies that can affect the course of yeast and mould infections through interaction with different cellular targets such as mannans, heat shock proteins, capsular polysaccharides, surface proteins, yeast killer toxin receptors which may be envisaged as potential vaccine candidates on their own. The paradigm that the complexity and isotype of protective, non protective and even disease-enhancing antibodies could account for the uncertainty of their role in immunity to fungi and the efficacy of antibodies is dependent on the relative contribution of each of them is now overcome by the technical feasibility and unlimited availability of reproducible human, humanized monoclonal and single chain antibodies as well as genetically engineered antibody fragments of defined specificity which may greatly strengthen the perspectives of antibody-mediated therapy even in individuals with defective immune responses.

The role played by antibodies (Abs) in the anticandidal defense has long been a matter of controversy, mostly due to the past inability to clearly define antigen specificity, the relationship between the type of immune response within the different settings of experimental and human candidiasis and, last but not least, a misunderstanding about the role of T helper cell in cell-mediated versus the humoral immunity. Contributory was also the lack of precise identification of virulence traits of the fungus which are the best candidates for a protective Ab response. In recent years, an impressive amount of experimental evidence, and also some clinical proof, have been generated which assign to Abs of defined specificity an important role in the anticandidal defense both at systemic and mucosal sites. Paradigmatic among them, Abs against defined virulence factors such as adhesins or aspartyl-proteinase enzymes, or against critical viability molecules such as β-glucan, have been detected or generated which hold great promise for immunotherapeutic interventions in humans.

Prevention of hematogenously disseminated candidiasis and mucocutaneous disease, including Candida vaginitis, through immunological approaches is appealing for the following reason. A long-acting and safe vaccine that protects against both serotypes of Candida albicans and other important species, such as C. tropicalis and C. glabrata, should significantly reduce the incidence of various forms of candidiasis by these etiologic agents. Through extensive experimentation on protective responses in experimental animals against Candida mannan components, others and we have evidence that antibodies specific for short-chain β-linked oligomannosides are protective against candidiasis. Although the mechanism of protection against vaginal infection requires further investigation, experimentally the ability of antibody to rapidly deposit high amounts of complement factor C3 onto the yeast cell wall is requisite for enhancing resistance against disseminated candidiasis.

Immunization of mice with a stress mannoprotein of >200 kDa from the cell wall of Candida albicans led to the production of monoclonal antibody (Mab) C7. The immunogen is a major target of secretory IgA and its expression is regulated by different environmental conditions including temperature, pH, glucose concentration and ammonium sulphate in the culture medium. Mab C7 reacted with a peptide epitope present in the >200 kDa antigen as well as in a number of antigens from the blastoconidium and germ tube cell wall, including enolase. In addition to its reactivity with C. albicans, Mab C7 also reacted with antigens present in C. krusei, C, tropicalis, C. glabrata, C. dubliniensis and C. lusitaniae, as well as in Cryptococcus neoformans, Scedosporium prolificans and Aspergillus fumigatus. Mab C7 exhibited four important biological activities, namely inhibition of adhesion of C. albicans to a variety of surfaces, inhibition of germination of C. albicans, direct candidacidal activity and direct tumoricidal activity. In tumor cells, Mab C7 reacted with nucleoporin Nup88, a reactivity that can be utilized for diagnostic and prognostic purposes.

Recent years have seen the development of the concept of combination therapy for treating severe fungal sepsis. The advantages of this approach are a potential improvement in patient survival and a reduction in the chance of resistance developing to each of the single agents. The disadvantage is that combining drugs may increase the chance of toxicity. Mycograb® is a genetically recombinant antibody against fungal heat shock protein 90 (hsp90) which is poised to become the mainstay of combination therapy. This paper presents data on how hsp90 is important to fungi and what role it might play in human disease with possible interactions with interleukin 6 and nitric oxide. There is discussion of preclinical data demonstrating synergy in vitro between Mycograb® and amphotericin B and caspofungin. The progress of Mycograb® through a Phase II pharmacokinetic study when used in escalating doses with a liposomal amphotericin B preparation has also been reviewed. The concepts behind a Phase II pivotal study, where Mycograb® or a placebo was given in combination with a liposomal amphotericin B drug for five days for the treatment of disseminated candidiasis are discussed.

The capsule of Cryptococcus neoformans, the principal virulence factor of this fungus, is composed primarily of polysaccharide. The predominant component of the polysaccharide capsule is glucuronoxylomannan (GXM), a compound with potent immunoregulatory properties. GXM is bound and internalized by natural immune cells affecting innate and subsequent adaptive immune response. The cellular pattern recognition receptors involved in GXM binding include toll-like receptor (TLR)4, CD14, TLR2, CD18, Fc gamma receptor II (FcγRII). This multiple cross-linking leads to a suppressive outcome that is arrested and even reversed by protective antibodies to GXM. This review analyzes the immunosuppressive effects induced by capsular material, considering its pattern recognition receptors, and dissects the mechanism of monoclonal antibody shifting to immunoactivation.

At first glance Cryptococcus neoformans appears an unlikely microbe to provide a new understanding of mechanisms of antibody-mediated immunity (AMI), because it is a facultative intracellular fungal pathogen for which the role of naturally acquired AMI in host defense is uncertain. However, numerous studies have now established that certain antibodies (Abs) against C. neoformans are protective in certain hosts. Studies with Abs to C. neoformans have provided new insights into AMI and generated new precedents with implications for other pathogens. The following concepts have emerged: 1) susceptibility to C. neoformans may be related to qualitative and quantitative aspects of the Ab response; 2) protective monoclonal Abs can be generated against pathogens even when the role of humoral immunity is uncertain; 3) Abs to C. neoformans mediate protection by immunomodulatory effects, thereby linking Ab efficacy to the overall host immune response; 4) Ab efficacy is critically dependent on fine specificity, which in turn is affected by immunoglobulin variable region usage, somatic mutation and constant region usage; 5) the efficacy of passive Ab therapy is a function of Ab dose and infecting innoculum, with lack of efficacy at the extremes of Ab concentration; 6) Ab-mediated toxicity resulting from antigen-Ab complex-induced release of platelet activating factor is isotype dependent. Observations with C. neoformans have stimulated a reappraisal of the role of humoral immunity for other pathogens and highlighted the limitations in current methods of assessing the role of Ab in host defense.

The host response to infection is the outcome of a complex interaction between a microbe and a host's innate and adaptive immune system. In this context, the role of antibody in the endemic mycoses is relatively poorly understood. Recently, a monoclonal antibody to a cell surface protein has been shown to be protective in a murine histoplasmosis model. The findings with Histoplasma capsulatum may provide a paradigm for antibody protection against endemic fungi. This paper reviews the recent data on protective antibody in histoplasmosis and previous data supporting a role for antibody in protective responses in other dimorphic fungi.

After several years of controversy, antibodies (Abs) are now believed to play an important role in the protection against fungal infections. Among them, recent data are strongly supporting the relevance of protective yeast killer toxin-like Abs (“antibiobodies”, KT-Abs), which are able to exert a direct microbicidal activity by mimicking a killer toxin (PaKT) and its interaction with cell wall receptors on susceptible cells essentially constituted by β-glucans. This review will focus on the implications of the yeast killer phenomenon, and, particularly, the occurrence and antimicrobial activity of protective antifungal KT-Abs, such as those produced during the course of experimental and natural infections caused by PaKT-sensitive microorganisms or produced by idiotypic vaccination with a PaKTneutralizing mAb. The strong therapeutic activity exerted against different experimental mucosal and systemic mycoses by monoclonal and recombinant microbicidal KT-Abs (either in their soluble forms or expressed on human commensal bacteria) as well as by a synthetic killer peptide (KP, an antibody fragment engineered from the sequence of a recombinant KT-Ab) will be discussed. The surprisingly wide antimicrobial spectrum of activity against eukaryotic and prokaryotic pathogenic agents, such as fungi, bacteria and protozoa, of these Abs and Ab-derived molecules suggests new potential strategies for transdisease anti-infective prevention and therapy.