The Casadevall Laboratory is interested in two fundamental questions: 1) How do microbes cause diseases; 2) How do hosts protect themselves against microbes? To address these broad questions the laboratory takes multidisciplinary approaches that span several areas of basic immunology and microbiology. Our research mainly focus on pathogenic microbes including Cryptococcus neoformans, Bacillus anthracis, and Mycobacterium tuberculosis. Some of our scientific interests are described below.

Cryptococcus neoformans

C. neoformans is the causative agent of cryptococcosis, affecting mostly immunocompromised patients, especially those with HIV infection, cancers, and organ transplant. The three well established virulence factors of C. neoformans include the capsule, melanin, and the ability to survive at human body temperature. Antifungal drugs are commonly used to treat fungal infections, but the problem of emerging drug-resistant strains has become more serious. Therefore, a better prevention and treatment such as vaccine is urgently needed.

Capsule structure and function

The capsule, consisting of glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoprotein, is one of the main virulence factors of C. neoformans. The size, structure, and physical property of the capsule can dramatically change depending on the growth environment, age of culture, and serotype. We study the composition and physical properties of GXM and GalXM in order to better understand the macromolecular structure of the capsule and to resolve key issues such as the identity of polysaccharide epitopes recognized by protective antibodies. We are also interested in the process of capsule growth, and how this capsule dynamics being translated into virulence against the host.

Melanin structure and function

Melanin production in C. neoformans is associated with virulence. Melanin is a pigment with an undefined chemical structure and tremendous physical stability. This pigment accumulates in the cell wall of C. neoformans. We are interested in understanding the fundamental biological process of how melanin in the cell wall is remodeled to allow growth and budding to occur. In addition, we are also studying how bacterial-fungal interactions, which may occur in the environment, cause melanization of C. neoformans, ultimately allowing resistance against amoeboid predators and mammalian hosts.

Antibody structure and binding specificity

The structure of the constant region of an antibody molecule can influence the function of the variable region including its binding specificity. We use a variety of spectroscopic techniques including X-ray crystallography to probe the structural and electronic properties of the constant region of the antibody against GXM, in order to reveal how conformational differences in antibody isotypes influence the binding specificity.

Quorum sensing

Quorum Sensing (QS) is a mechanism of communication between microbial cells, mediated by molecules (QSM) that are accumulated during cell growth. When the QSMs reach a certain threshold concentration, they induce the entire population to cooperate in behaviors such as bioluminescence, antibiotic production, sporulation, biofilm formation, and virulence. QS is well known in bacteria, but eukaryotic QS was unknown until the recent discovery of QSMs in Candida albicans. We have thus decided to investigate the presence of cell density-dependent behavior in C. neoformans and to check the possible effects of cell-density molecules in growth, virulence, and host-pathogen interactions.

Biofilm formation

Biofilms are communities of microorganisms that attach to surfaces and allow them to survive in new and sometimes hostile environments. The ability of microbes to generate biofilms on indwelling and prosthetic medical devices has become a troublesome medical problem. C. neoformans biofilms allow for increased antifungal resistance. We are currently developing an in vivo model for C. neoformans biofilm formation. In addition, we are also investigating how antibodies may protect the host by preventing C. neoformans biofilm formation.

Phagosome extrusion

C. neoformans has the capacity to survive and replicate within the phagolysosome of macrophage cells. After multiplying, multiple fungal cells have been observed to be released, a process termed extrusion, from the immune cell and continue to survive. It has also been observed that this process of extrusion often involves a giant phagosome, suggesting that homotypic phagosome fusion occurs prior to extrusion. Our research focuses on the identification of genes that may be involved in phagosome fusion, and determining whether silencing these genes prevents subsequent extrusion. We are also interested in studying the possible role of annexins which function to bring cellular membranes in close proximity to each other to facilitate fusion in the process of extrusion. 

Origin of virulence in pathogenic fungi

The mechanism of acquiring and maintaining virulence by C. neoformans is unknown. The evolution of virulence traits is being studied in three different systems. The interactions of C. neoformans with the free-living soil amoeba Acanthamoebae castellanii and the slime mold Dictyostelium discoideum are being characterized. These interactions help us understand the environmental survival strategy of C. neoformans and relate to the emergence of fungal virulence for humans. In particular, we are interested in the evolution of virulent traits in environmental microbes that do not require a host to complete their life cycle. We are working on the selective forces that drive heat tolerance acquisition (a necessary virulence factor for mammalian pathogens) within saprobic fungi in the Tremellales, the taxa that includes the human pathogenic cryptococci C. neoformans and C. gattii.

Bacillus anthracis

Anthrax, caused by B. anthracis, occurs when anthrax spores are ingested, inhaled, or enter the body cutaneously. The spores can survive at extremes temperatures and conditions, be easily dispersed, which make them a possible agent of biological warfare. Motivated by the fact that the current vaccine against anthrax has limitations including its temporary immunity and undesirable side effects, our laboratory is interested in devising better antibody-based countermeasures to protect against anthrax.

Vaccine development

Bacillus anthracis capsule is a linear polymer of gamma-D-glutamic acid which contributes to the pathogenicity of the bacteria allowing it to evade the host-immune response.  Since the current vaccine is poorly immunogenic and transient, there is an urgent need to re-evaluate the current vaccine used for treatment of anthrax and expand the immunity conferred by these available anthrax vaccines.  To that end, we generated a set of monoclonal antibodies to the capsule which will be used for studies of capsule synthesis and for passive antibody therapy by targeting the capsule.  These antibodies will be used to gain a better understanding of the potential of humoral immunity against a complex antigen.  We will also gain greater insight into the relationship between antibody structure and function for an unusual antigen composed of a polymerized D-amino acid.  We anticipate that the studies will produce new information on basic antibody structures and gene usage that is of fundamental importance for vaccine design and the treatment of anthrax.   

Antibody-Fc receptor interactions

Monoclonal antibodies have become an important therapeutic strategy in toxin neutralization. Understanding the role of Fc receptor is important for the currently available anthrax vaccine, which is believed to mediate protection by eliciting antibodies that neutralize the protective antigen component of anthrax toxin, yet is poorly immunogenic and does not protect all hosts against experimental anthrax. Recently we found that antibody isotype was pertinent for protecting against toxin and that the efficacy of a monoclonal antibody can be enhanced by changing its constant region. These results may suggest the presence of optimal combinations of antibody specificity, constant region usage and Fc receptor type that would differ depending on the particular antibody and toxin pair.

Mycobacterium tuberculosis

M. tuberculosis is the causative agent of tuberculosis, causing millions of new cases of active disease per year. Tuberculosis if not treated can be fatal. While Bacille Calmette-Guerin (BCG) has been used as vaccine for years, it does not completely prevent people from getting tuberculosis. Hence there is an unquestioned need for new and effective vaccines to prevent tuberculosis. Our research focuses include the pathogenesis of M. tuberculosis, and the development of a better conjugate vaccine against tuberculosis.

Generation of conjugate vaccine

Current vaccine strategies are largely focused on improving the BCG vaccine or developing new antigenic preparations that elicit cell-mediated immunity.  To our knowledge there are no serious efforts to develop a vaccine that mediates protection through antibody-based immunity despite the fact that most, if not all, licensed vaccines against infectious diseases mediate protection by eliciting antibody immunity.

Mycobacterial membrane vesicles

Many intracellular bacterial pathogens use membrane vesicles (MVs) as an alternative secretion mechanism that delivers ligands that can be recognized by host cells.  These MVs have been extensively studied in Gram negative bacteria. Certain MVs can carry additional virulence factors, such as toxins, adhesins or immunomodulatory compounds that are important for pathogenesis. For mycobacteria, there is a report of MV production by Mycobacterium ulcerans, which implies that vesicular transport and delivery systems are also found among mycobacteria.  We have found MVs in many mycobacterium species, including the medically important Mycobacterium bovis BCG attenuated vaccine strain and virulent M. tuberculosis.  Since mycobacterial MVs are enriched in lipids and proteins previously known to be involved in the subversion of the immune response of the host, we want to investigate whether these vesicles can contributing to virulence and pathogenesis in M. tuberculosis infection.