As I asked in the previous part: Why is the capsule a virulence factor? What special properties make it a key player in the process by which Cryptococcus neoformans (CN) causes disease?
Well, while acapsular mutants (lack external capsule, but contain GalXM on the cell wall), avirulent in normal hosts, may still cause disease in severely immunocompromised hosts, the burden of virulence associated with CN has traditionally fallen on the capsule, because of the extra-ordinary effects that it has on the immune system (quick reminder: these studies were largely done with what we now know as the ‘exopolysaccharide’, i.e. the shed capsular material).
Interestingly, CN appears to have the ability to modify the size and structure of the capsule in response to various stimuli, including the micro-environment of the target organ, stage of infection, stage of fungal cellular growth, and so forth. In mouse infection models, the capsule size increases rapidly (within hours) following infection; strains isolated from severely immunocompromised individuals (such as HIV/AIDS patients) often show large capsular diameter. The capsule composition (including ratio of various sugars, and arrangement of repeating units) may change at various stages during the infection. Since the host immune system presumably encounters the capsule first, these changes likely modulate how the host immune components react against CN.
Cryptococcal cells of various capsule sizes (all images under same magnification)
Let me try to summarize some of the major deleterious effects which the capsular polysaccharides (including the exopolysaccharide) have on the host immune system. Needless to say, these effects are crucial in helping CN establish the disease in the host. [Note: The host immunity plays an unwitting role in the establishment of the cryptococcal disease, but we shall discuss those factors later.]
- Capsule interferes with normal functioning and immune functions of epithelial cells: Two types of epithelial cells (lining the surface of cavities) encounter inhaled CN first, the bronchial epithelium (inner lining of the branches of trachea, the air-tube) and the alveolar epithelium (inner lining of the air sacs – pockets of air – where exchange of gases takes place). Both epithelial cell types are capable of responding to the capsule, initiate a signal via release of messenger proteins called cytokines, and call for backup in form of an immune cell called neutrophil, which can kill microbes. However, the capsule suppresses cytokine release from bronchial epithelial cells; in alveolar epithelium, however, the capsule causes CN to be internalized in the epithelial cell, which kills the host cells. The resulting host cell damage likely allows CN to cross the epithelial barrier to reach inside the lung tissue.
- Capsule helps CN evade capture by largely inhibiting phagocytosis: Phagocytic cells (cells which engulf and kill foreign substances, including invading microbes), such as the Macrophages in the alveolar pockets of the lung tissue, are among the first line of defence against invading microbial pathogens. The engulfment is initiated either by direct interaction between macrophages and the microbe, or via components of the humoral (i.e. non-cellular) immune system (antibodies and/or complements) of the host. Apart from killing the microbe, this process also presents bits and pieces of microbial material (‘antigen presentation’) to another immune cell, the helper T-lymphocytes (‘T-cells’), which ultimately allow the antibody-producing B-lymphocytes (‘B-cells’) to produce specific antibodies recognizing the microbe, as well as create immunologic memory. The cryptococcal capsule helps the microbe evade capture (thereby, also interfering with antigen presentation) by largely inhibiting the phagocytic process
- Capsule also interferes with additional immune mechanisms that enable phagocytosis: Even in presence of capsule, antibodies and/or complements (mentioned above) can bind to the capsule and/or cell wall, and enable phagocytosis. However, a large capsule may effectively mask the binding sites for these immune proteins.
- Capsule allows intracellular parasitism of phagocytosed CN: Even when the first-line immune mechanisms can successfully help Macrophages engulf the cryptococcal cell, the capsule actively interferes with their antimicrobial properties, so that internalized CN is not killed, but thrives, with the ability to spread to other cells, tissues, and organs.
- Capsule aids dissemination of CN: Along with other mechanisms (including various cryptococcal enzymes), the capsule allows the spread of CN from lungs to the rest of the body. The microbe is able to change the composition of the capsule, presumably in order to adapt to different micro-environments. The structure and composition of the capsule are also important for the ability of CN to cross various physiological barriers to reach the target tissue.
- Capsule is able to induce immunological unresponsiveness to CN (a.k.a. immune paralysis): The polysaccharide is known to be able to inhibit antibody production by B-cells, and growth and proliferation of T-cells; this is achieved via inhibition of antigen presentation by phagocytes (mentioned above), modulation of the production and release of certain cytokines, as well as induction of a certain subset of T-cells whose secreted products make other T-cells unresponsive. In addition, the direct binding of the polysaccharide to certain receptors (known as FCRγII) in various phagocytic cells leads to a profound immune suppression. The capsule also interferes with the functions of various other immune effector cells, such as neutrophils. Most of these effects have been described for the GXM component (see previous part) of the capsule, but GalXM and the mannoprotein components have also been implicated in similar effects. GXM and GalXM both can induce antigen-presenting phagocytic cells and T-cells to commit suicide (via a process called ‘apoptosis’).
Click to embiggen: Various types of cryptococcal colony morphologies on artificial medium; it has been hypothesized that changes in colony morphology, controlled genetically, may be associated with changes in structure and composition of the capsule.
CN is abundantly present in the environment (especially in endemic areas, in association with certain trees and birds); therefore, one enduring mystery is the reason why it would need to evolve to make a capsule with such high virulence potential in mammalian hosts (primates, quadrupeds, as well as aquatic mammals). Extensive work done in the laboratory of Arturo Casadevall at the Albert Einstein College of Medicine in New York, as well as elsewhere, have shown that, in the environment, CN has to interact with various hosts, including certain amoebas, slime mould, nematode worms and insects – many of which can kill the microbe; the capsule affords protection against those marauders in a way that is remarkably similar to its interaction with the mammalian immune cells. Pathogenicity and virulence of CN in mammalian hosts may, therefore, be a consequence of that process.
In the next installment, I shall describe some different types of the cryptococcal organism, including their clinical significance.
Review of the Cryptococcal Capsule, by Oscar Zaragoza et al. Advances in Applied Microbiology, 2009, 68:133-216.