Cytotoxic effects of natural killer cells have no significant role in controlling infection with the intracellular protozoon Eimeria vermiformis

Unraveling the role of natural killer cells in leishmaniasis

NK cells are known as frontline responders that are efficient in combating several maladies as well as leishmaniasis caused by Leishmania spp. As such they are being investigated to be used for adoptive transfer therapy and vaccine. In spite of the lack of antigen-specific receptors at their surface, NK cells can selectively recognize pathogens, accomplished by the activation of the receptors on the NK cell surface and also as the result of their effector functions. Activation of NK cells can occur through interaction between TLR-2 expressed on NK cells and. LPG of Leishmania parasites. In addition, NK cell activation can occur by cytokines (e.g., IFN-γ and IL-12) that also lead to producing cytokines and chemokines and lysis of target cells. This review summarizes several evidences that support NK cells activation for controlling leishmaniasis and the potentially lucrative roles of NK cells during leishmaniasis. Furthermore, we discuss strategies of Leishmania parasites in inhibiting NK cell functions. Leishmania LPG can utilizes TLR2 to evade host-immune responses. Also, Leishmania GP63 can directly binds to NK cells and modulates NK cell phenotype. Finally, this review analyzes the potentialities to harness NK cells effectiveness in therapy regimens and vaccinations.

Innate Lymphoid Cells in Protection, Pathology, and Adaptive Immunity During Apicomplexan Infection

Apicomplexans are a diverse and complex group of protozoan pathogens including Toxoplasma gondii, Plasmodium spp., Cryptosporidium spp., Eimeria spp., and Babesia spp. They infect a wide variety of hosts and are a major health threat to humans and other animals. Innate immunity provides early control and also regulates the development of adaptive immune responses important for controlling these pathogens. Innate immune responses also contribute to immunopathology associated with these infections. Natural killer (NK) cells have been for a long time known to be potent first line effector cells in helping control protozoan infection. They provide control by producing IL-12 dependent IFNγ and killing infected cells and parasites via their cytotoxic response. Results from more recent studies indicate that NK cells could provide additional effector functions such as IL-10 and IL-17 and might have diverse roles in immunity to these pathogens. These early studies based their conclusions on the identification of NK cells to be CD3–, CD49b+, NK1.1+, and/or NKp46+ and the common accepted paradigm at that time that NK cells were one of the only lymphoid derived innate immune cells present. New discoveries have lead to major advances in understanding that NK cells are only one of several populations of innate immune cells of lymphoid origin. Common lymphoid progenitor derived innate immune cells are now known as innate lymphoid cells (ILC) and comprise three different groups, group 1, group 2, and group 3 ILC. They are a functionally heterogeneous and plastic cell population and are important effector cells in disease and tissue homeostasis. Very little is known about each of these different types of ILCs in parasitic infection. Therefore, we will review what is known about NK cells in innate immune responses during different protozoan infections. We will discuss what immune responses attributed to NK cells might be reconsidered as ILC1, 2, or 3 population responses. We will then discuss how different ILCs may impact immunopathology and adaptive immune responses to these parasites.

Structural and functional dissection reveals distinct roles of Ca2+-binding sites in the giant adhesin SiiE of Salmonella enterica

The giant non-fimbrial adhesin SiiE of Salmonella enterica mediates the first contact to the apical site of epithelial cells and enables subsequent invasion. SiiE is a 595 kDa protein composed of 53 repetitive bacterial immunoglobulin (BIg) domains and the only known substrate of the SPI4-encoded type 1 secretion system (T1SS). The crystal structure of BIg50-52 of SiiE revealed two distinct Ca²⁺-binding sites per BIg domain formed by conserved aspartate or glutamate residues. In a mutational analysis Ca²⁺-binding sites were disrupted by aspartate to serine exchange at various positions in the BIg domains of SiiE. Amounts of secreted SiiE diminish with a decreasing number of intact Ca²⁺-binding sites. BIg domains of SiiE contain distinct Ca²⁺-binding sites, with type I sites being similar to other T1SS-secreted proteins and type II sites newly identified in SiiE. We functionally and structurally dissected the roles of type I and type II Ca²⁺-binding sites in SiiE, as well as the importance of Ca²⁺-binding sites in various positions of SiiE. Type I Ca²⁺-binding sites were critical for efficient secretion of SiiE and a decreasing number of type I sites correlated with reduced secretion. Type II sites were less important for secretion, stability and surface expression of SiiE, however integrity of type II sites in the C-terminal portion was required for the function of SiiE in mediating adhesion and invasion.

The interaction of Salmonella enterica with polarized epithelial cells depends on the function of SiiE, a 595 kDa adhesin containing 53 repeats of a bacterial immunoglobulin (BIg) domain. SiiE is secreted and surface-expressed by a cognate type I secretion system (T1SS). We found that BIg domains contain amino acid (aa) residues forming binding sites for Ca²⁺ ions. Two types of Ca²⁺-binding sites can be distinguished, termed type I and type II sites. We performed a structural and functional dissection of Ca²⁺-binding sites of SiiE. After mutation of aa residues forming type I and/or type II Ca²⁺-binding sites, we investigated the secretion, surface expression and function as adhesin for interaction with polarized epithelial cells of the SiiE variants. We found that Ca²⁺-binding sites are critical for supporting the secretion of SiiE. Integrity of type I sites in any position of SiiE is essential for efficient secretion and surface expression. In contrast integrity of type II sites is less important for secretion. However, loss of type II in the C-terminal, most distal portion of SiiE ablated SiiE-mediated adhesion, while loss of the type II sites in middle or N-terminal portions of SiiE had less or no effect on SiiE function. We propose a novel mechanism of Ca²⁺-dependent secretion and conformational fine tuning of SiiE as a large T1SS substrate with a central role in the interaction of S. enterica with host cells.

A selective review of advances in coccidiosis research

Coccidiosis is a widespread and economically significant disease of livestock caused by protozoan parasites of the genus Eimeria. This disease is worldwide in occurrence and costs the animal agricultural industry many millions of dollars to control. In recent years, the modern tools of molecular biology, biochemistry, cell biology and immunology have been used to expand greatly our knowledge of these parasites and the disease they cause. Such studies are essential if we are to develop new means for the control of coccidiosis. In this chapter, selective aspects of the biology of these organisms, with emphasis on recent research in poultry, are reviewed. Topics considered include taxonomy, systematics, genetics, genomics, transcriptomics, proteomics, transfection, oocyst biogenesis, host cell invasion, immunobiology, diagnostics and control.

Role of T lymphocytes and cytokines in coccidiosis

Development of a vaccine for avian coccidiosis has been hampered by lack of understanding of the various components of the host immune system leading to protective immunity. Clear understanding of the cellular dichotomy in cytokine production in mice and the availability of immunological reagents, as well as gene knock-out mice, now makes in-depth immunological study in this species feasible. From studies of various parasitic infection models in mice, it is becoming clear that complex regulation by cytokines is involved in host immunity. Furthermore, the studies in mice clearly indicated an important role of various effector mechanisms involving T lymphocytes, macrophages, natural killer (NK) cells and cytokines in resistance to coccidiosis. In comparative studies of coccidiosis in chickens, in-vivo and in-vitro studies revealed that interferon-gamma, tumor necrosis factor and transforming growth factor-beta are induced following Eimeria infection. Depletion studies revealed the importance of CD8+TCR-alpha-beta+ T lymphocytes in host protective immunity to avian coccidiosis. Taken together, studies in mice and chickens are providing a better understanding of the role of effector cells and soluble factors which control immune responses to Eimeria parasites.

Genetic analysis of the essential components of the immunoprotective response to infection with Eimeria vermiformis

The immune responses generated after infection with Eimeria spp. are complex, include both cellular and humoral components, and lead to protection against re-infection. To facilitate the rational development of the next generation of anticoccidial vaccines it is important that the nature of the immunoprotective response against infection with Eimeria spp. is determined. In this brief report we discuss results that were obtained using a combination of genetic and cellular approaches to dissect the essential immune effector components that operate against infection with Eimeria vermiformis. Mice rendered deficient of immune function by targeted gene disruption at a variety of immune loci represent an integral component of our studies and include those with targeted gene disruption at loci that encode the B- and T-cell receptors (BCR, TCR), antigen presentation molecules and immune-effector molecules. Our studies demonstrated that TCR-alpha-beta + T cells are essential for immunoprotection during both primary and secondary infection. Moreover, during primary infection the major effector cell type is a population of major histocompatibility complex class II-restricted, interferon-gamma-producing TCR-alpha-beta T cell consistent with a T helper 1 phenotype. In addition, there is a supplementary role for another class of cells (presumably T cells) that are restricted to either non-classical antigen presentation molecules or classical major histocompatibilty complex class I loaded via an atypical pathway. Mice with a deficiency in interleukin-6 were slightly more susceptible to primary infection than intact animals, consistent with the reported effects of interleukin-6 upon the generation of T helper 1-type responses in vivo. In terms of the host response to re-infection, TCR-alpha-beta T cells were essential for immunity, but the requirement for specific cell subsets and effector mechanisms was much less stringent. Mice deficient in gamma-delta T cells, classical major histocompatibility complex class I, non-classical antigen presentation pathways, the cytokines interferon-gamma, interleukin-4, interleukin-6 and the cytolytic effector molecules perforin or FasL were completely immune to secondary infection. Moreover, major histocompatibility complex class II-deficient I-A-beta-/- mice were capable of mounting a substantial response to secondary infection, manifest by a 95% reduction in oocyst output compared with primary infection. These data have important consequences for the development of immune intervention strategies and indicate that vaccine development may be targeted toward the generation of a wider range of effector mechanisms than those that operate during primary infection.

Role of natural killer cells in innate resistance to protozoan infections

Natural killer cells are now recognized as major effectors of innate resistance to protozoan parasites. The principal mechanism by which they control the growth of these pathogens is indirect, involving cytokine production rather than cytolytic activity. Recent studies have identified a series of positive and negative signals provided by cytokines and cellular interactions which regulate protozoa-induced natural killer cell function.

T-cell receptor gamma--delta lymphocytes and Eimeria vermiformis infection

The role of T-cell receptor gamma--delta T lymphocytes in coccidiosis was examined by determining the course of infection with Eimeria vermiformis in BALB/c mice depleted of gamma--delta lymphocytes by treatment with GL3 monoclonal antibody. The replication of the parasite in primary infections was not greatly, or consistently, affected by this treatment, and there was no correlation between the extent of depletion of small intestinal intraepithelial lymphocytes and the number of oocysts produced. The resistance of immunized mice to challenge was not compromised by depletion of intraintestinal epithelial lymphocytes when their depletion was effected at the time of primary infection and/or administration of the challenge inoculum. Thus, T-cell receptor gamma--delta T lymphocytes do not appear to be crucial to the establishment, or the control, of primary infection with E. vermiformis and are not principal mediators of the solid immunity to challenge that this infection induces.