Cell-Mediated Immune Response

Tuberculosis is a significant global disease today, so understanding its origins and history is important. It is primarily a lung infection and is transmitted by infectious aerosols from person to person, so a high population density encourages its spread. The causative organism is Mycobacterium tuberculosis, an obligate pathogen in the M. tuberculosis complex that also contains closely related species, such as Mycobacterium bovis, that primarily infect animals. Typical bone lesions occur in about 5% of untreated infections. These can be recognized in historical and archaeological material, along with nonspecific paleopathology such as new bone formation (periostitis), especially on ribs. Based on such lesions, tuberculosis has been found in ancient Egypt, pre-Columbian America, and Neolithic Europe. The detection of M. tuberculosis ancient DNA (aDNA) by using PCR led to the development of the new field of paleomicrobiology. As a result, a large number of tuberculosis cases were recognized in mummified tissue and bones with nonspecific or no lesions. In parallel with these developments, M. tuberculosis cell wall lipid biomarkers have detected tuberculosis suggested by paleopathology and confirmed aDNA findings. In well-preserved cases, molecular typing has identified M. tuberculosis lineages and genotypes. The current interest in targeted enrichment, shotgun sequencing, and metagenomic analysis reveals ancient mixed infections with different M. tuberculosis strains and other pathogens. Identification of M. tuberculosis lineages from samples of known age enables the date of the emergence of strains and lineages to be calculated directly rather than by making assumptions on the rate of evolutionary change.

Immunohistochemical studies identified serous cells of the glandular acini as the cells responsible for production of salivary histatins. Histatins exhibit fungicidal activity against several Candida species, Aspergillus fumigatus, some strains of Saccharomyces cerevisiae, and Cryptococcus neoformans. Studies of levels of salivary histatins in vivo show large intersubject variation in both the concentrations of histatins and their rates of degradation. The total concentration of histatins in whole saliva is balanced between secretion of new proteins and removal of “older” proteins by degradation. Endocytosis was initially suggested as a means of histatin cellular entry based upon the observation that bafilomycin, an inhibitor of endosomal acidification, significantly decreased antifungal activity. Confocal imaging of C. albicans cells showed that some histatin 5 was localized to the vacuole but that cells containing only vacuolar histatin were viable. The cell wall of C. albicans is a thick multilayered structure of glucans, chitin, and mannoproteins that protects cells from osmotic stress and maintains structural integrity. Animal and human clinical studies to evaluate histatins as topical agents in prevention of gingivitis reported therapeutic efficacy without adverse side effects. The major requirements for effective use of salt-insensitive fungicidal peptides are selective and specific binding and uptake by candidal cells, efficacy at low concentrations that allow rapid eradication of yeast pathogens within the ionic strength of saliva, and minimal fungal resistance.

Serologic prevalence data indicate that toxoplasmosis is one of the most common infections of humans throughout the world. Toxoplasmosis can be categorized into four groups: (i) acquired in the immunocompetent patient; (ii) acquired or reactivated in the immunodeficient patient; (iii) congenital; and (iv) ocular. Acquired infection with Toxoplasma in immunocompetent individuals is generally an asymptomatic infection. Secretions, excretions, body fluids, and tissues are potential specimens for direct observation of parasites but are generally unrewarding. The most important advantage of the immunoglobulin M (IgM) capture enzyme immunoassay (EIA) compared to indirect fluorescent antibody (IFA) is the increased detection of congenital infections: the IgM enzyme-linked immunosorbent assay (ELISA) was positive for 73% of serum samples from newborn infants with proven congenital toxoplasmosis, whereas only 25% of the same serum samples were found positive by an IFA IgM test. The currently recommended drugs work primarily against the actively dividing tachyzoite form of T. gondii and do not eradicate encysted organisms (bradyzoites). T. gondii infection is one of the most common parasitic infections worldwide and, in most instances, is of little clinical significance. The laboratory will need to use additional testing in an attempt to define the timing of infection in the case of pregnant women or the presence of actively replicating parasites in the case of the fetus, neonate, or immunosuppressed host.

Signal transduction cascades are utilized by all organisms to convey signals perceived at the cell surface to effectors within the cell. These enzymatic signaling cascades are important in the pathogenesis of many infections, including cryptococcosis. This chapter summarizes the significance and functional interactions involved in the cell wall integrity, phospholipase, and calcineurin signaling pathways for the establishment of Cryptococcus neoformans virulence. The fungal Plc enzymes referred to in this review preferentially hydrolyze phosphatidylinositol (PI)-based substrates within the cryptococcal cell and affect multiple cellular functions, including the secretion of (phospholipase B ) Plb1. It was found that the Plb1 MW could be as high as 125 kDa due to extensive asparagine N-linked glycosylation, which is responsible for at least 30% of the MW of Plb1 and essential for its activity. It was recently demonstrated that PI-PLC1 (Plc1) regulates cryptococcal virulence, acting in part through interactions with the Pkc/Mpk1 cell wall integrity pathway. In contrast to Plcs from higher eukaryotes, Plcs from the parasite Trypanosoma brucei preferentially hydrolyze the glycosylphosphatidylinositol (GPI) anchor of variant surface glycoprotein or GPI biosynthetic intermediates, in addition to PI, but not the phosphorylated intermediates, despite their localization to the peripheral cytoplasmic face of intracellular vesicles. Metabolic labeling studies performed in S. cerevisiae implicated a Plc enzyme and a secondary-acting protease in hydrolysis of the GPI anchor of certain proteins in the plasma membrane, resulting in their subsequent localization in the cell wall. ScPlc1, the only Plc1 in S. cerevisiae, like CnPlc1, lacks a secretory signal leader peptide.

The most common of the human diseases caused by fungi are the opportunistic fungal infections that occur in patients with defective immunity. This chapter attempts to position the new findings on acquired immunity and fungi within the conceptual framework of a two-component antifungal response that includes resistance and tolerance. Resistance and tolerance are two types of host defense mechanisms that increase fitness in response to fungi. The enzyme indoleamine 2,3-dioxygenase (IDO) and tryptophan metabolites contribute to immune homeostasis by inducing Tregs and taming heightened inflammatory responses. The new entry, the Th17 pathway, playing an inflammatory role previously attributed to uncontrolled Th1 cell reactivity and Tregs and capable of fine-tuning protective antimicrobial immunity in order to minimize harmful immune pathology, have become an integral component of the immune response to fungi. In their capacity to induce Tregs and inhibit Th17, IDO and kynurenines pivotally contribute to cell lineage decision in experimental fungal infections and reveal an unexpected potential in the control of inflammation, allergy, and Th17-driven inflammation in these infections. Serological and skin reactivity surveys indicate the development of acquired cell-mediated immunity (CMI) to fungi. A number of clinical observations suggest an inverse relationship between interferon (IFN)-γ and interleukin (IL)-10 production in patients with fungal infections. The implication for IDO in immunoregulation in fungal infections has several important implications.

This chapter examines several mechanisms by which two well-known protozoan pathogens, one extracellular and one intracellular, successfully evade host immunity. It discusses how one protozoan parasite has evolutionarily addressed few problems by displaying a highly ordered molecular surface coat that serves largely to protect therypanosome plasma membrane from immunological assault. Many successful parasites exhibit antigenic variation to avoid immune elimination during infection. The most well-known example of immune evasion by parasites is antigenic variation by the African trypanosome, which for nearly a century has provided the classical paradigm for microbial antigenic variation as a means of escaping host immunity. In addition to an apparent absence of variant surface glycoprotein (VSG) C-terminal peptide processing by antigen-presenting cell (APCs) following primary exposure, there is evidence that the processing of parasite antigens may be more broadly regulated during progressive infection. Leishmania parasites infect host macrophage cells during their life cycle. Development of effective cell-mediated immune responses to these organisms requires that infected macrophages induce significant Th1 cell activation against leishmanial antigens. Parasitic protozoa regulate almost every aspect of host innate and adaptive immunity, and have evolved multiple mechanisms that permit passive and active evasion of host resistance. These mechanisms include not only the expression of variant antigens and specialized surface coats (African trypanosomes), but also the modulation of antigen expression and signaling pathways of the cells that they infect or interact with (trypanosomesand Leishmania spp.), effectively subverting or suppressing cellular mechanisms that can affect parasite survival.

Mumps virus is classified as a member of the order Mononegavirales, family Paramyxoviridae, subfamily Paramyxovirinae, and genus Rubulavirus. Penetration of mumps virus virions into target cells begins with binding of the hemagglutininneuraminidase (HN) surface glycoprotein to sialic acid-containing receptors on the cell surface. Humans can be experimentally infected by inoculation of mumps virus onto the nasal or buccal mucosa, suggesting that natural infection results from airborne spread of droplets of respiratory secretions from infected to susceptible individuals. Mumps virus can establish persistent infection in neuronal cell lines and chronic central nervous system (CNS) infection in animal models, but it does not routinely cause chronic infections in humans. In the suckling hamster model, mumps virus CNS infection is associated with the development of stenosis of the sylvian aqueduct and with granular ependymitis. The majority of mumps patients with CNS involvement will have signs of meningeal irritation but no evidence of cortical dysfunction. The pathogenesis of mumps-related arthritis is uncertain; virus has not been isolated from joint fluid, and there is no evidence of immune complex deposition. Although clinical experience with this assay is currently limited, reverse transcriptase (RT)-PCR is likely to become the diagnostic method of choice for mumps virus CNS infection. Therapy of patients with uncomplicated mumps consists of conservative measures to provide symptomatic relief, such as analgesics, antipyretics, rest, and hydration. Symptomatic measures to alleviate the pain and swelling of mumps orchitis include bed rest, scrotal support and opioid analgesics.

This chapter discusses the fundamental role of phagocytes, with specific emphasis on macrophages and dendritic cells (DCs), as potent cytokine producers and responders, establishing them as primary effector cells as well as regulators of innate and adaptive immune responses. It draws attention to the role of T-cell-derived cytokines produced during adaptive Th1 and Th2 responses, IFN-γ and IL-4/IL-13, respectively, and their fundamental role in modulating macrophage and DC function. The chapter also discusses the cellular and molecular mechanisms that enable macrophages and DCs to express complex cytokine patterns in response to pathogens, which are key to mounting effective innate and downstream adaptive immune responses. The cellular response induced by the engagement of PRRs is often referred to as the “innate” activation of phagocytes, in contrast to the “classical” and “alternative” activation of macrophages by cytokines. IL-4 and IL-13 also induce intracellular enzymes that are implicated in cell recruitment and granuloma formation. Therefore, the distinctive macrophage phenotype induced by the Th2-type cytokines IL-4 and IL-13, also referred to as alternatively activated macrophages, plays an important role in dampening classical cell-mediated immune responses while reprogramming the response toward an appropriate type that promotes an efficient eradication of extracellular pathogens. Recognition of microorganisms or their products initiates complex signaling pathways in these cells that determine the pattern of cytokines produced and the subsequent effector response, critical for the eradication of pathogens with minimum damage to the host.

This chapter focuses on recent developments in one’s understanding of how different components of the cell envelope from virulent mycobacteria, in particular Mycobacterium tuberculosis, interact with each stage of innate and adaptive immune responses. Human dendritic cells express an additional C-type lectin, dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN), which is not present on macrophages but serves as a major receptor for M. tuberculosis on dendritic cells (DCs). Cell wall components of mycobacteria bind to these pathogen recognition receptors on DCs and macrophages, and initiate the host response to infection. The biological relevance of TLR activation by cell wall components has been examined by mycobacterial infection of gene-deficient mice. Inactivation of protein kinase G (PknG) by gene disruption or chemical inhibition resulted in delivery of pathogenic mycobacteria to lysosomes and mycobacterial killing, and conversely, expression of PknG in nonpathogenic prevented phagosomal maturation. The induction of adaptive immunity to mycobacteria and activation of infected macrophages by IFN-γ can overcome the maturation arrest of mycobacteria containing phagosomes. CD1 proteins, which are antigen presenting molecules encoded by genes located outside of the major histocompatibility complex (MHC), recognize nonpeptide lipid or glycolipid structures, including components of mycobacteria. The containment of mycobacterial infection requires the formation of granulomas (or tubercles), which are nodular aggregations of lymphocytes, macrophages, and epithelioid cells. In human M. tuberculosis infection, the propensity of trehalose dimycolate (TDM) and other cell wall components to promote granuloma formation in association with a chronic T-cell response leads to caseating granulomas, which may erode into airways.

Leishmaniasis refers to a diverse group of diseases, with the spectrum of disease depending on the infecting species. Leishmania major, L. tropica, L. aethiopica, and, rarely, L. infantum cause cutaneous disease in the Old World; disease manifestations include nodular and ulcerative skin lesions. Lesions usually occur on exposed parts of the body such as the face, hands, feet, arms, and legs; uncommon sites include the ears, tongue, and eyelids. Lymphatic spread may occur in L. major infections, with subcutaneous nodules in a linear distribution and regional lymphadenopathy; if the initial lesion is on the hand, this clinical presentation may resemble sporotrichosis. The majority of AIDS patients present with the classical picture of visceral leishmaniasis (VL), but asymptomatic cutaneous leishmaniasis (CL), mucocutaneous leishmaniasis, “disseminated” CL (DCL), and post-kala-azar dermal leishmaniasis can be seen. The RK39 strip test is ideal for rapid reliable field diagnosis of VL. The test has high sensitivity and specificity; however, it remains positive long after treatment. L. chagasi causes subclinical infections and American visceral leishmaniasis (AVL), which is potentially fatal if not treated, and it has recently been associated with atypical cutaneous leishmaniasis (ACL) in Central America, particularly in Honduras. PCR will probably become the test of choice, as with other types of leishmaniasis, but the practicality and cost issues mean that routine methods will continue to be the most likely approach, particularly since the majority of cases are seen in more remote rural areas.