Interrelations among Multiple Metrics of Immune and Physiological Function in a Squamate, the Common Gartersnake (Thamnophis sirtalis)

The field of ecoimmunology has made it clear that individual and ecological contexts are critical for interpreting an animal's immune response. In an effort to better understand the relevance of commonly used immunological assays, we tested how different metrics of immunity and physiological function were interrelated in naturally parasitized individuals of a well-studied reptile, the common gartersnake (Thamnophis sirtalis). Overall, we found that bactericidal ability, an integrative measure of innate immunity, was often correlated with more specific immunological and physiological tests (lysis and oxidative stress) but was not related to tissue-level inflammation that was determined by histopathology. The only hematological metric that correlated with tissue-level inflammation was the prevalence of monocytes in blood smears. Finally, using histological techniques, we describe natural parasitism throughout the organ systems in these individuals, finding that neither the presence nor the burden of parasite load affected the physiological and immune metrics that we measured. By performing comprehensive assessments of physiological and immune processes, we are better able to draw conclusions about how to interpret findings from specific assays in wild organisms.

Evaluation of immune functions in captive immature loggerhead sea turtles (Caretta caretta)

Sea turtles face numerous environmental challenges, such as exposure to chemical pollution and biotoxins, which may contribute to immune system impairment, resulting in increased disease susceptibility. Therefore, a more thorough assessment of the host's immune response and its susceptibility is needed for these threatened and endangered animals. In this study, the innate and acquired immune functions of sixty-five clinically healthy, immature, captive loggerhead sea turtles (Caretta caretta) were assayed using non-lethal blood sample collection. Functional immune assays were developed and/or optimized for this species, including mitogen-induced lymphocyte proliferation, natural killer (NK) cell activity, phagocytosis, and respiratory burst. Peripheral blood mononuclear cells (PBMC) and phagocytes were isolated by density gradient centrifugation on Ficoll-Paque and discontinuous Percoll gradients, respectively. The T lymphocyte mitogens ConA significantly induced lymphocyte proliferation at 1 and 2 μg/mL while PHA significantly induced lymphocyte proliferation at 5 and 10 μg/mL. The B lymphocyte mitogen LPS significantly induced proliferation at 1 μg/mL. Monocytes demonstrated higher phagocytic activity than eosinophils. In addition, monocytes exhibited respiratory burst. Natural killer cell activity was higher against YAC-1 than K-562 target cells. These optimized assays may help to evaluate the integrity of loggerhead sea turtle's immune system upon exposure to environmental contaminants, as well as part of a comprehensive health assessment and monitoring program.

Regulation of innate immunity in tilapia: activation of nonspecific cytotoxic cells by cytokine-like factors

Exposure of tilapia (Oreochromis niloticus) to water temperatures of 10-15 degrees C for 3-5 min produces physiological stress responses characterized by immediate phenotypic and immunological changes. In the present study, this general stress response was utilized as a model system to study innate immunity mediated by soluble factors and cytotoxic cells. Acute innate cytotoxic responses of nonspecific cytotoxic cells (NCC) in the peripheral blood (PBL), anterior kidney (AK) and spleen (SPL) were measured. Following temperature stress, the levels of NCC activity depended on the presence of soluble factors and on the cell compartments from which the NCC were obtained. NCC from PBL of stressed tilapia had 30x or greater cytotoxic activity compared to nonstressed PBLs from controls. NCC activity from the AK and SPL of stressed tilapia was lower than controls. Flow cytometric analysis of NCC in each tissue showed that increased cytotoxicity was not produced by increased numbers of NCC. To determine the mechanism of amplification of cytotoxicity, NCC from nonstressed tilapia were passively treated with serum from temperature stressed tilapia. Serum containing the "stress activated serum factor" (SASF) passively increased naive NCC cytotoxicity (from PBL) 3-4 fold. The cytotoxic cell response was inhibited by addition of anti-NCC monoclonal antibody 5C6. These data indicated that NCC are (at least one of) the target cells for SASF. SASF required only 15 min pre-incubation with naive NCC to activate cytotoxicity. Activation was nonreversible and concentration dependent. Pretreatment of NCC with SASF reduced the assay time required to amplify target cell cytotoxicity from 12-24 h to 6 h. SASF amplification of NCC cytotoxicity was not restricted by different histological types of target cells. Determination of select physical/chemical properties of SASF revealed: complete heat inactivation of cytotoxicity amplification following 55 degrees C and 65 degrees C pretreatment; SASF was thermostable at room temperature to 45 degrees C for 15 min; and freeze-thaw treatment reduced but did not completely remove amplification activity. The molecular weight range of SASF activity was identified in a 50-100 kDa fraction obtained by differential dialysis. SASF appears to be a protein sensitive to trypsin digestion.

Cell adhesion and the immune system: a case study using earthworms

In the earthworm's immune system, cell adhesion, which occurs by putative receptors on leukocytes, is essential after recognition of self vs. non-self. Confrontation with foreign antigens is a normal event in the environment, replete with microbial pathogens that pose a threat to survival. To better understand what happens when an effector cell first recognizes a foreign target followed by its adhesion to it, isolated leukocytes, in sufficient quantities to be subjected to various analyses, have been extremely beneficial. In vitro approaches when accompanied by biochemical, immunological, and molecular technologies, have opened up new vistas concerning the immune response of earthworms and other invertebrates. The most recent discovery includes the preliminary identification of cell differentiation (CD) markers that play vital roles in recognitive and adhesive events. Certain leukocyte effectors show characteristics of natural killer (NK) cells that may act differently depending upon their source, whether autogeneic, allogeneic, xenogeneic, or expressed under normal or varying environmental conditions including exposure to xenobiotics. At the level of earthworm evolution, there is apparently a dissociation of phagocytosis from the process of killing by NK-like effectors. There are at least three future challenges. First, it is essential to determine the precise nature of the CD markers with respect to their molecular structure. Second, once their molecular and biochemical characteristics have been defined, the role of these markers in cellular and humoral mechanisms must be clarified in order to define effector cell products and resulting immune responses. Third, there is a need to differentiate between the several lytic factors that have been found in earthworms with respect to molecular structure, and biochemical and functional characterization.

Mechanisms of cellular cytotoxic innate resistance in tilapia (Oreochromis nilotica)

Mechanisms of innate cytotoxic immunity in tilapia (O. nilotica) were measured by characterization of the activity, distribution and functions of nonspecific cytotoxic cells (NCC). Active cytotoxic cells were obtained from anterior kidney. spleen and peripheral blood whereas nonlytic but anti-NCC monoclonal antibody 5C6 positive cells were obtained from tilapia liver. Thymocytes were not cytotoxic and were mab 5C6+. Unfractionated anterior kidney cells were 6% mab 5C6+ and had very low cytotoxicity of HL-60 target cells. Percoll (45.5%) purified NCC were 44% mab 5C6+ and had 35% HL-60 cytotoxicity (160:1 E:T ratio). Transformed mouse and human target cells were tested for sensitivity to NCC lysis. HL-60, U937, K562, IM-9 and NC-37 human targets were lysed by NCC. YAC-1 targets were insensitive to lysis. The killing of HL-60 targets by tilapia NCC was inhibited by mab 5C6. Experiments to determine optimal conditions for the cytotoxicity assay revealed that tilapia required 15-20h for optimum lysis of targets. Incubation at 37 C produced the highest cytotoxicity. The proliferative competence of Percoll purified anterior kidney cells was determined. A significant increase in in vitro uptake of tritiated thymidine by anterior kidney cells occurred following stimulation by mab 5C6, Con-A, PMA and calcium ionophore A23187. Purified spleen cells also produced significant increased uptake of tritiated thymidine following in vitro activation with PMA and mab 5C6, but not Con-A. These studies indicated that NCC may provide innate cytotoxic immunity similar to that provided by the NCC of catfish.

Molecular characterization of a protozoan parasite target antigen recognized by nonspecific cytotoxic cells

The target cell antigen(s) on tumor cells and on protozoan parasites recognized by NK and nonspecific cytotoxic cells (NCC) has not yet been specifically identified. NCC may be the teleost equivalent of NK cells and IL-2-activated NK cells. A ligand recognized by NCC has been identified. It is expressed on both protozoan parasites and mammalian tumor target cells. In the present study, a protozoan parasite antigen (NK target antigen/NKTag/p46) was purified from Tetrahymena pyriformis and the entire amino acid sequence was deduced from cDNA. Soluble and purified NKTag inhibited NCC lysis of human and mouse transformed target cells. Homology comparisons using Swissprot database revealed that NKTag is a novel protein. Molecular weight computation of the deduced sequence demonstrated that NKTag is a 48.17-kDa protein containing 422 amino acids with relatively high percentages of tyrosine and serine residues. Expression of NKTag on various mammalian tumor target cells, normal tissue, and T. pyriformis was determined using anti-multiple antigenic peptide (MAP) monoclonal antibody (mab) 22A12 [generated against an N-terminal 20-mer (aa 61-80) of p46]. This mab bound to tissue-cultured and tumor cells (YAC-1, IM-9, NC-37, MOLT-4, and U937) with low levels of binding to fish, mouse, and equine cells. Studies were also done to determine if purified and iodinated NKTag bound specifically to NCC. Binding was saturable and specific. These data provide evidence that NCC recognize a target cell ligand which is found on both protozoan and tumor cells. This may provide an explanation as to how NCC (including activated NK cells) recognize a vast array of targets in the absence of haplotype recognition and in spite of a diverse species of origin.

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.

In vitro spontaneous cytotoxic activity against mammalian target cells by the hemocytes of the solitary ascidian, Ciona intestinalis

Blood cell-mediated cytotoxic activity against mammalian target cells by the hemocytes of the solitary ascidian Ciona intestinalis was investigated in vitro by fluorochromasia. Salt-conditioned target cells were labeled with carboxyfluorescein diacetate and challenged with mixed and separated hemocytes. The assay provided optimal conditions for the functioning of the effector hemocytes while maintaining low background leakage from the target cells. Comparison of different hemocyte populations, separated by density gradient centrifugation, revealed that only cell bands containing the phagocytic and nonphagocytic amoebocytes exhibited cytotoxicity. Experiments to characterize cytolysis demonstrated that activity increased with the effector to target cell ratio, occurred within 15 min, and was maximal at an incubation temperature of 20 degrees C. Both human (K562) and mouse [YAC-1, P815, WEHI (3B) and L929] target cell lines were killed by the ascidian effector hemocytes. This paper demonstrates a population of nonspecific cytotoxic effector cells in the blood of C. intestinalis that are able to spontaneously kill a range of mammalian targets in vitro.

Natural killer (NK) cells in domestic animals: phenotype, target cell specificity and cytokine regulation

A comprehensive review of NK cells in animals of veterinary medical importance has not been previously published. However, these cells have a high level of immunological/medical relevance due to their role in tumour cell destruction and B-cell regulation, as well as their inhibitory activities against various parasites and bacteria. In the present review, NK cells from agriculturally important animals are characterized. Cell phenotype descriptions have shown that for each species, unique (i.e. non-cross-reactive with anti-human CD antibodies) and different monoclonals are required to identify NK cells. These cells lyse certain tumour and virus transformed target cells and, as might be expected from the diverse species compared in this review, analysis of the tissue distribution of NK cells gives highly varied results. NK cell morphology differs in these species from small agranular to large granular lymphocytes. The final area considered relates to studies on the effects of cytokines on NK function and to research identifying which cytokines (if any) are produced by NK cells during activation responses. The largest quantity of available data concerns functional and descriptive studies in these animals. However, it is appropriate to consider this research as a starting point on which to build comparative and molecular studies of the roles of NK cells in immunosurveillance and immunoregulation.

Expression of an evolutionarily conserved function associated molecule on sheep, horse and cattle natural killer cells

Natural killer (NK) cells are large granular lymphocytes that lyse a wide variety of transformed and virally-infected target cells without prior exposure to antigen, and without restriction by major histocompatibility complex antigens. Although NK cells have been identified in a variety of mammalian species, how NK cells recognize antigen and trigger lysis is unknown. Recently, monoclonal antibodies made against NK-like cells from teleost fish were shown to react with NK cells from humans and rats, and to inhibit their cytolytic activity. The role of this apparently evolutionarily conserved function-associated molecule (FAM) has been further investigated utilizing a variety of domesticated farm animal species. It was observed that the anti-FAM mAb reacted specifically with peripheral blood lymphocytes isolated from sheep, horses and cattle. Further, the anti-FAM mAb inhibited NK cell lytic activity in each of these species. Finally, the anti-FAM mAb was found to inhibit conjugate formation between NK and target cells, implying that the FAM was involved in antigen recognition by NK cells in each of these species. In conclusion, it appears that NK cell function is mediated by an evolutionarily conserved FAM in a wide variety of species.

Expression of a novel function-associated molecule on cells mediating spontaneous cytolysis in swine

In the present study, we have analyzed the effects of the anti-FAM mAb 5C6, on endogenous NK cell and lymphokine-activated killer (LAK) cell activity of swine. We report that the anti-FAM mAb 5C6 binds to fresh and cultured lymphocytes with spontaneous cytotoxic activity from swine peripheral blood. In conjunction, the anti-FAM mAb 5C6 inhibits swine cytolysis against K562 target cells. Inhibition was found to be mediated by blocking of conjugate formation. The level of spontaneous killing and mAb inhibition was correlated with the number of swine cells staining positive with the mAb 5C6 as assessed by flow cytometry.

Invertebrate immunity: another viewpoint

All vertebrates and invertebrates manifest self/non-self recognition. Any attempt to answer the question of adaptive significance of recognition must take into account the universality of receptor-mediated responses. These may take two forms: (1) rearranging, clonally distributed antigen-specific receptors that distinguish in the broadest sense between self and non-self, and non-self A from non-self B, latecomers on the evolutionary scene; (2) pattern recognition receptors, the earliest to evolve and still around, necessitating the requirement for induced second signals in T- and B-cell activation. Either strategy need not force upon invertebrates the organization, structure and adaptive functions of vertebrate immune systems. Thus, we can freely delve into the unique aspects of the primitive immune mechanisms of invertebrates. In contrast, using the opposite strategy which is still problematic, i.e. linking invertebrate and vertebrate defence, seems to give us an approach to universality that might eventually reveal homologous kinship.