Differential distribution of gamma delta T-cell receptor lymphocyte subpopulations in blood and spleen of young and adult cattle

Comparative Analysis of Maternal Colostrum and Colostrum Replacer Effects on Immunity, Growth, and Health of Japanese Black Calves

Maternal colostrum (MC) is an important source of nutrients and immune factors for newborn calves. However, when colostrum is unavailable or of poor quality, a colostrum replacer (CR) may be a suitable alternative to MC. As stock-raising farmers must make informed decisions about colostrum feeding management, this study was conducted to determine the effect of feeding MC versus CR on the promotion of immunological status, growth, and health in pre-weaned Japanese black (JB) calves. Sixteen newborn JB calves were fed MC after birth, and 16 JB calves were fed CR. For the MC group, the numbers of γδ T cells, CD4+ cells, CD8+ cells, CD4+CD8+ cells, B cells, and MHC class II+ cells were significantly higher compared with the CR group. Furthermore, the expression levels of interleukin (IL)-1β-, IL-2-, and interferon-γ (IFN-γ)-encoding mRNAs were significantly higher in the MC group compared with the CR group. A lower incidence of disease in 1-month-old calves and higher carcass weight in the MC group were observed compared with the CR group. These results suggest that CR activates the immune system delayed in calves compared with MC. MC increases populations of various immunocompetent cells, which can reduce infection rates and improve body weight gain.

γδ T cells in artiodactyls: Focus on swine

Vaccination is the most effective medical strategy for disease prevention but there is a need to improve livestock vaccine efficacy. Understanding the structure of the immune system of swine, which are considered a γδ T cell "high" species, and thus, particularly how to engage their γδ T cells for immune responses, may allow for development of vaccine optimization strategies. The propensity of γδ T cells to home to specific tissues, secrete pro-inflammatory and regulatory cytokines, exhibit memory or recall responses and even function as antigen-presenting cells for αβ T cells supports the concept that they have enormous potential for priming by next generation vaccine constructs to contribute to protective immunity. γδ T cells exhibit several innate-like antigen recognition properties including the ability to recognize antigen in the absence of presentation via major histocompatibility complex (MHC) molecules enabling γδ T cells to recognize an array of peptides but also non-peptide antigens in a T cell receptor-dependent manner. γδ T cell subpopulations in ruminants and swine can be distinguished based on differential expression of the hybrid co-receptor and pattern recognition receptors (PRR) known as workshop cluster 1 (WC1). Expression of various PRR and other innate-like immune receptors diversifies the antigen recognition potential of γδ T cells. Finally, γδ T cells in livestock are potent producers of critical master regulator cytokines such as interferon (IFN)-γ and interleukin (IL)-17, whose production orchestrates downstream cytokine and chemokine production by other cells, thereby shaping the immune response as a whole. Our knowledge of the biology, receptor expression and response to infectious diseases by swine γδ T cells is reviewed here.

Gene characterization and expression of the γδ T cell co-receptor WC1 in sheep

Sheep are known to express the hybrid co-receptor/pattern recognition receptor WC1 on their γδ T cells but details of the ovine WC1 multigenic array and gene expression were unknown. Annotation of the sheep genome assembly (Oar_rambouillet_v1.0) yielded 15 complete and 42 partial WC1 genes predicted to code for six different protein structures. RT-PCR amplification of the most distal scavenger receptor cysteine rich (SRCR) domain known as a1, which serves as the gene signature, from genomic and cDNA templates verified the majority of annotated genes. As for cattle and goats, sheep a1 domain sequences included WC1.1 and WC1.2 types. A unique ovine gene, WC1-16, had multiple SRCR a-pattern domains in tandem similar to one found in goats. Intracytoplasmic domains of WC1 transcripts had splice variants that may affect signal transduction. The larger number of WC1 genes in sheep and differences in structures and splice variants relative to cattle could have implications in expression patterns and engagement of γδ T cells by pathogens or vaccine constructs.

Goat γδ T cells

Goats are important food animals and are disseminated globally because of their high adaptability to varying environmental conditions and feeding regimes that provide them with a comparative advantage. Productivity is impacted by infectious diseases; this then contributes to societal poverty, food insecurity, and international trade restrictions. Since γδ T cells have been shown to have vital roles in immune responses in other mammals we reviewed the literature regarding what is known about their functions, distribution in tissues and organs and their responses to a variety of infections in goats. It has been shown that caprine γδ T cells produce interferon-γ and IL-17, are found in a variety of lymphoid and nonlymphoid tissues and constitute a significant population of blood mononuclear cells. Their representation in tissues and their functional responses may be altered concomitant with infection. This review summarizes caprine γδ T cell responses to Brucella melitensis, Fasciola hepatica, Mycobacterium avium paratuberculosis, caprine arthritis encephalitis virus (CAEV), and Schistosoma bovis in infected or vaccinated goats. Caprine γδ T cells have also been evaluated in goats infected with M. caprae, Ehrilichia ruminantium, Haemonchus contortus and peste des petits ruminants (PPR) virus but found to have an unknown or limited response or role in either protective immunity or immunopathogenesis in those cases.

WC1+ and WC1neg γδ T lymphocytes in intestinal mucosa of healthy and Mycobacterium avium subspecies paratuberculosis-infected calves

Mucosal surfaces such as the gastrointestinal tract, and skin are the front line of host defence and immunity against many pathogens. Gamma delta (γδ) T lymphocytes preferentially localize to the mucosal surfaces in several species including cattle, and are thought to play crucial roles in immunosurveillance and host defence, particularly against mycobacteria. Many γδ T cells are present in young calves, which is the period when calves are thought to be initially exposed to Mycobacterium avium subspecies paratuberculosis (Map). The role of mucosal γδ T cells in cattle, especially during host-pathogen interactions during early pre-clinical phases of infectious disease remains unclear. The purposes of this study were to investigate and characterize WC1⁺ and WC1^neg γδ  T cell subsets in various segments of the gastrointestinal (GI) tract of young calves, and then to examine γδ  T cell subsets in the distal small intestine of calves after experimental intestinal Map infection by direct Peyer's patch inoculation. We show that in healthy calves, the relative proportion of γδ T cells is constant throughout the GI mucosa, though the ileum has significantly more γδ T cells. In the distal intestine, γδ T cells are mainly WC1^neg and primarily located within the lamina propria of the jejunum and ileum. In Map-infected intestine, there are higher numbers of γδ T cells in the lamina propria and a greater proportion of WC1⁺ cells within the epithelial layer compared to control calves. While WC1^neg γδ T cells preferentially localize to the distal small intestine of healthy calves, WC1⁺ γδ T cells are increased in the intestinal mucosa during Map infection, which is suggestive of effector cell function. Further, spectral microscopy and flow cytometry in tandem will lead to improved understanding of the functions of these cells during health and disease.

Bovine peripheral blood WC1+ and WC1neg γδ T lymphocytes modulate monocyte-derived macrophage effector functions during in vitro Mycobacterium avium subspecies paratuberculosis infection

The importance of bovine γδ T lymphocytes during anti-mycobacterial immunity is recognized; however, the role of major subsets of γδ T lymphocytes (WC1⁺ and WC1^neg) in this process remains unclear. We investigated how WC1⁺ and WC1^neg γδ T lymphocyte subsets of calves modulate monocyte-derived macrophage (MDM) functions during Map infection in vitro. To achieve this, Map-infected or uninfected MDMs from young calves were co-cultured with autologous WC1⁺ or WC1^neg γδ T lymphocytes. Our data indicate that WC1⁺ and WC1^neg γδ T lymphocytes of young calves modulate effector functions of MDMs with respect to Map killing, CD11b and MHC-II expression. We observed differences in IFN-γ production and CD25 expression on γδ T lymphocyte subsets, as well as MDM expression of CD1b when in contact with WC1^neg γδ T lymphocytes.

Bovine WC1(+) γδ T lymphocytes modify monocyte-derived macrophage responses during early Mycobacterium avium subspecies paratuberculosis infection

Following Mycobacterium avium subspecies paratuberculosis (Map) infection, some calves are apparently able to successfully clear the pathogen whereas others become persistently infected; however the reasons for this remain unknown. The importance of innate immunity, and in particular the role of γδ T lymphocytes, during early anti-mycobacterial immune response is recognized but specific mechanisms remain incompletely characterized. The objective of this study was to investigate how bovine WC1(+) γδ T lymphocytes mediate macrophage function during early Map infection. To achieve this objective, Map-infected monocyte-derived macrophages (MDMs) were co-cultured either in direct contact with, or separated by a semi-permeable membrane from, autologous WC1(+) γδ T lymphocytes. Nitrites, IL-17A, IFN-γ, IL-4 and IL-10 from cell culture supernatants were measured. Expression of CD25 on WC1(+) γδ T lymphocytes, expression of MHC-I and MHC-II on MDMs and the viability of Map recovered from MDM cultures 72h after Map infection were also assessed. Map viability was significantly reduced when WC1(+) γδ T lymphocytes were co-cultured in direct contact with Map-infected MDMs. Both MDMs and WC1(+) γδ T lymphocytes generated increased concentrations of IFN-γ and IL-4 in our system, and MDM/WC1(+) γδ T lymphocyte synergism was identified for IFN-γ production. MDMs but not WC1(+) γδ T lymphocytes were a significant source of IL-17A. The presence of WC1(+) γδ T lymphocytes was associated with higher expression of MHC-I on MDMs and increased concentration of nitrites in supernatants 72h after Map infection. In conclusion, this study showed that WC1(+) γδ lymphocytes had differential effects on Map-infected macrophages in vitro.

Immature T cell neoplasms in three young cattle

Immature T cell neoplasms in three young Holstein cattle with neoplastic involvement of the thymus are described. Case 1, with a precursor T lymphoblastic leukemia (calf form of leukosis), was an 86-day-old female calf. The leukemia was characterized by replacement of the bone marrow and spleen by leukemia cells, but preservation of epithelial frameworks throughout the thymus. The other two neoplasms were thymic γδ T cell lymphomas, which were observed in a 246-day-old steer (case 2) and a 16-month-old heifer (case 3). Histological examination revealed obliteration of the normal thymic architecture and stromal fibrosis, with the spleen and liver far less severely affected than in case 1. There were cytological differences bewteen the tumors in case 1 and cases 2 and 3. Additionally, WC1 and CD8 were expressed only in the latter. Thus, the leukemia and these lymphomas should be regarded as independent disease entities on the basis of histological and immunohistochemical characteristics.

Beyond mice and men: environmental change, immunity and infections in wild ungulates

In the face of rapid environmental change, anticipating shifts in microparasite and macroparasite dynamics, including emergence events, is an enormous challenge. We argue that immunological studies in natural populations are pivotal to meeting this challenge: many components of environmental change--shifts in biotic assemblages, altered climate patterns and reduced environmental predictability--may affect host immunity. We suggest that wild ungulates can serve as model systems aiding the discovery of immunological mechanisms that link environmental change with parasite transmission dynamics. Our review of eco-immunological studies in wild ungulates reveals progress in understanding how co-infections affect immunity and parasite transmission and how environmental and genetic factors interact to shape immunity. Changes in bioavailability of micronutrients have been linked to immunity and health in wild ungulates. Although physiological stress in response to environmental change has been assessed, downstream effects on immunity have not been studied. Moreover, the taxonomic range of ungulates studied is limited to bovids (bighorn sheep, Soay sheep, chamois, musk oxen, bison, African buffalo) and a few cervids (red deer, black-tailed deer). We discuss areas where future studies in ungulates could lead to significant contributions in understanding the patterns of immunity and infection in natural populations and across species.

Comparison of the peripheral blood leukocyte population between Japanese Black and Holstein calves

Japanese black (JB) calves have greater susceptibility to infectious diseases compared to Holstein (Hol) calves. In order to clarify the differences in cellular immune status between JB and Hol calves, the leukocyte population and lymphocyte proliferative ability were analyzed. In total 200 healthy calves, 1 day to 14 weeks of age, were examined: 105 JB and 95 Hol calves. Lower numbers in peripheral blood and percentage in peripheral blood mononuclear cells of CD3(+)TcR1-N12(+) T cells and major histocompatibility complex class-II(+)CD14(-) B cells were observed in the JB compared to the Hol. The percentage of TcR1-N12(+)CD25(+) T cell in the JB was significantly lower than that of the Hol at 4-6, and 8-10 weeks. Interleukin (IL)-2 sensitivity in the JB was lower than that in the Hol, and significant differences were observed in age groups of 6-8 weeks and 10-14 weeks. These findings indicated that the lower numbers of γδ T cells and B cells in the JB compared to the Hol might be associated with the specificity of the immune systems in JB calves.

Experimental infection of a bovine model with human isolates of Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium subsp. paratuberculosis (Map), the etiologic agent of Johne's disease (JD) in ruminants, has been implicated in the pathogenesis of Crohn's disease (CD) in humans. We developed a bovine ileal cannulation model to facilitate comparison of the immune response to Map and the mechanisms of pathogenesis in cattle and humans. Initial studies showed a T cannula could be maintained for up to a year in calves without inducing inflammation or adversely affecting intestinal function. Map introduced through the cannula established a persistent low level of infection without inflammation. Infection elicited an immune response to Map antigens detectable by flow cytometry. Further studies now show the cannulation model can be used with cows during the later stage of infection, affording access to the target tissue at all stages of infection. The studies also revealed no difference in infectivity or immunogenicity of isolates of Map obtained from cattle or humans with CD. Comparison of the immune response to Map during the early and late stages of infection using PCR, flow cytometry and QRT-PCR, showed the immune response early in the disease process is dominated by CD4 T cells. A CD8 response is delayed but comparable at later stages of infection. Genes for pro-inflammatory cytokines IFN-γ and the recently identified genes encoding IL-17 and IL-22 are up regulated in infected animals. These findings reveal that both human and bovine isolates of Map can establish infection and induce similar immune responses in a bovine model. They also reveal the cytokine responses elicited in cattle are similar to those implicated in CD pathogenesis.

Dynamics of bovine spleen cell populations during the acute response to Babesia bovis infection: an immunohistological study

The spleen is a critical organ in defence against haemoparasitic diseases like babesiosis. Many in vitro and ex vivo studies have identified splenic cells working in concert to activate mechanisms required for successful resolution of infection. The techniques used in those studies, however, remove cells from the anatomical context in which cell interaction and trafficking take place. In this study, an immunohistological approach was used to monitor the splenic distribution of defined cells during the acute response of naïve calves to Babesia bovis infection. Splenomegaly was characterized by disproportionate hyperplasia of large versus small leucocytes and altered distribution of several cell types thought to be important in mounting an effective immune response. In particular, the results suggest that the initial crosstalk between NK cells and immature dendritic cells occurs within the marginal zone and that immature dendritic cells are first redirected to encounter pathogens as they enter the spleen and then mature as they process antigen and migrate to T-cell-rich areas. The results of this study are remarkably similar to those observed in a mouse model of malarial infection, suggesting these dynamic events may be central to the acute response of naïve animals to haemoparasitic infection.

Peripheral blood mononuclear cell population changes associated with age and Mycobacterium avium subspecies paratuberculosis infection in red deer (Cervus elaphus)

In this study we aimed to document the maturation of the immune system in red deer and investigate the hypothesis that an immature immune system may predispose young red deer (Cervus elaphus) to an increased susceptibility to Johne's disease. Peripheral blood mononuclear cell populations were analysed using flow cytometry techniques to monitor changes associated with age and severity of infection with Mycobacterium avium subspecies paratuberculosis in red deer. The percentage of cells expressing cell population markers CD4, CD8, WC1, gammadelta TCR, CD14 and B-B4 as well as the cell activation markers CD25, CD44, ACT1 and ACT31 were analysed in relation to age and M. avium subsp. paratuberculosis infection status and disease severity. Significant changes in both cell surface markers and cell activation markers were observed as animals matured irrespective of their response to infection. The levels of CD4 and CD8 increased with age and the levels of B-B4, WC1, CD14 and gammadelta TCR decreased similar to previous studies in cattle. No differences were observed in cell surface markers or cell activation markers in relation to M. avium subsp. paratuberculosis infection status and disease severity. This research demonstrates that peripheral blood cell populations change with age in red deer and suggests that studies of cell surface markers and activation markers in peripheral blood samples do not provide information on the age-related susceptibility to Johne's disease.

The bovine spleen: interactions among splenic cell populations in the innate immunologic control of hemoparasitic infections

Over the past several years, innate immunity has been recognized as having an important role as a front-line defense mechanism and as an integral part of the adaptive immune response. Innate immunity in cattle exposed to hemoparasites is spleen-dependent and age-related. In this review, we discuss general aspects of innate immunity and the cells involved in this aspect of the response to infection. We also provide examples of specific splenic regulatory and effector mechanisms involved in the response to Babesia bovis, an important tick-borne hemoparasitic disease of cattle. Evidence for the regulatory and effector role of bovine splenic monocytes and DC both in directing a type-1 response through interaction with splenic NK cells and γδT-cells will be presented.

Bovine WC1(-) gammadeltaT cells incubated with IL-15 express the natural cytotoxicity receptor CD335 (NKp46) and produce IFN-gamma in response to exogenous IL-12 and IL-18

The gammadeltaT cells of ruminants are believed to participate in innate immunity and have been described with regulatory, inflammatory and cytotoxic functions. Here we describe a subset of CD3(+) TcR1(+) WC1(-)gammadeltaT cells expressing CD335 (NKp46), classically associated with CD3(-) natural killer (NK) cells, as a consequence of incubation with IL-15. This population, undetectable at the time of collection, developed after 2 week of IL-15 culture from splenic leukocytes (SPL) reaching greater than 50% of the total gammadeltaT cells. However, they did not grow well from peripheral blood leukocytes (PBL). Splenic gammadeltaT cells positively selected by magnetic separation prior to incubation with IL-15 and analyzed by flow cytometry, consistently yielded CD3(+) cells expressing CD335. These cells arose from the CD335(-)gammadeltaT cell population suggesting that the new population represents up-regulation of CD335 by gammadeltaT cells. CD335 mRNA expression from sorted IL-15-incubated SPL CD335(+)gammadeltaT cells or NK cells exceeded that of CD335(-)gammadeltaT cells. Incubation with IL-12 and IL-18 increased the number of CD335(+)gammadeltaT cells in both the PBL and SPL fractions as compared to controls or IL-12 or IL-18 alone. In addition, CD335(+)gammadeltaT cells demonstrated a robust ability to produce IFN-gamma in response to exogenous IL-12 and IL-18. Taken as a whole, we describe a new phenotypically distinct bovine gammadeltaT cell sub-population capable of participating in type 1 immune responses.

Workshop cluster 1+ gammadelta T-cell receptor T cells from calves express high levels of interferon-gamma in response to stimulation with interleukin-12 and -18

Gammadelta T-cell receptor(+) T lymphocytes are an important element of the innate immune system. Early production of interferon (IFN)-gamma by gammadelta T cells may have a role in linking innate and adaptive immune responses and contribute to T helper-1 bias. We investigated the role of cytokines in the activation and induction of IFN-gamma secretion by bovine workshop cluster 1(+) (WC1(+)) gammadelta T cells. The effects of culture with interleukin (IL)-12, IL-18, IL-15 and IL-2 were investigated; these cytokines are known to influence murine and human gammadelta T cells. We report that bovine WC1(+)gammadelta T cells are synergistically stimulated by IL-12 and IL-18 to secrete large quantities of IFN-gamma. Neonatal calves were shown to have significantly higher numbers of circulating WC1(+)gammadelta T cells than adult animals. In addition, the response of peripheral blood WC1(+)gammadelta T cells was significantly higher in neonatal calves compared with adult animals. However, in adult animals the response of lymph node WC1(+)gammadelta T cells to IL-12/IL-18 was more pronounced than that of peripheral blood WC1(+)gammadelta T cells. We hypothesize that the induction of IFN-gamma secretion from WC1(+)gammadelta T cells by IL-12 and IL-18 is likely to be an important element of the innate response to pathogens such as Mycobacterium bovis. The high numbers of WC1(+)gammadelta T cells in neonatal calves, and their inherent ability to respond to inflammatory cytokines, could be a key factor in the enhanced responses seen in calves to BCG vaccination.

Lymphocyte subpopulations and neutrophil function in calves during the first 6 months of life

Fifteen clinically healthy calves were sampled every week during the first 5 weeks of life and thereafter every month until the age of 6 months. The percentages and absolute values of CD4+, CD8+ gammadelta TCR+ and WC1+ T cells, CD21+ B cells and NKp46+ NK cells were determined by flow cytometry, and the expression of the interleukin-2 receptor alpha chain (CD25) was measured to assess the level of activation of the lymphocyte subpopulations. Neutrophil phagocytosis, respiratory burst and bactericidal activity were measured in five different neutrophil function assays. Most of the parameters examined reached a stable level during the first 6 months of life. The proportions of CD4+ and CD8+ lymphocytes remained relatively stable during the study period, while there was a moderate decrease in the relative percentage of gammadelta T cells from birth to approximately 5 months of age. However, the absolute numbers of gammadelta T cells per millilitre of blood remained stable throughout the study period and did not display significant variation with age. The percentage of cells expressing the B-cell maturation marker CD21 increased significantly over the first 5 months of life. The proportion of NK cells showed substantial variation during the study. Marked differences in the relative proportions of the lymphocyte subpopulations were noted between the individual calves, and the individual ranking of the animals was largely maintained over time. CD25 expression was detected on a mean of 6.6% of the CD4+ cells, while a lower percentage of the other lymphocyte subpopulations expressed this receptor. Phagocytic activity was demonstrated in approximately 90% of the neutrophils, and this proportion remained stable during the entire study period, while respiratory burst activity showed a moderate decrease during the first 2 months of life. The present study shows that the T-cell subpopulations are present in peripheral blood of calves at levels comparable with adult values, while the B-cell population increases significantly with age. The decrease in the relative percentage of gammadelta T cells appears to be attributable to an increase in the absolute numbers of CD4+ and CD21+ cells, rather than a change in absolute gammadelta T-cell numbers. Furthermore, the results indicate that the neutrophilic granulocytes are functional and able to mount an effective response in young calves from the first week of life.

The association of sub-clinical paratuberculosis with the fertility of Greek dairy ewes and goats varies with parity

Our cross-sectional study investigated the association of sub-clinical Mycobacterium avium subsp. paratuberculosis (MAP) infection with failing to produce a live offspring the season of lambing/kidding (November 2001 to January 2002) before testing (in April-May 2002), in four dairy-sheep and/or goat flocks in Greece (369 animals >or=1.5-year-old). From each selected animal 10 ml of blood and 10 g of feces from the rectum were obtained. The harvested sera were tested for antibodies to MAP with a commercial ELISA test kit; the feces were cultured on Herrold's egg-yolk medium supplemented with mycobactin J and antibiotics. An animal was considered sub-clinically infected when found either seropositive or culture positive. The true prevalence of sub-clinically infected animals, adjusted for the sensitivity and specificity of the parallel testing, was 14% (0.1-28%) and 35.9% (9.2-62.7%) in sheep and goats, respectively. The association of fertility of sheep and goats with sub-clinical paratuberculosis was investigated in random-effects logistic models. Sub-clinically infected animals (compared to uninfected) had OR for live offspring the previous year of 5.4 for parity <4, OR=0.05 for parity >6, and a non-significant OR for the middle parity category.

Effect of Dietary Administration of Bananas on Immunocytes in F1 Hybrid Calves

The effect of dietary administration of bananas on immunocytes in calves was investigated. Twenty Fl hybrid calves were used in this study (treated group n=10, control group n=10). Banana (2 g/kg BW) was administered to the calves for 5 days. Leukocyte subsets were examined on days 0, 5, 10, and 15. The numbers CD3+, (CD3+)CD45R-, and (CD3+)TcR+ cells significantly increased between day 0 and day 5 in the treated group (P<0.01), and were significantly higher on day 5 in the treated group relative to the control group (P<0.05). These data showed that feeding banana to calves increased T-lymphocytes, suggesting it might be possible to enhance protective functions against infections.

Response of T Lymphocytes From Previously Infected Calves to Recombinant Cryptosporidium parvum P23 Vaccine Antigen

We had previously demonstrated that a Type-1-like immune response involving interferon-gamma expression in lamina propria lymphocytes accompanied by IgG2 subclass fecal antibodies to Cryptosporidium parvum p23 emerged in gut mucosa of calves recovering from cryptosporidiosis. Because a recombinant p23 had been shown to protect calves from cryptosporidiosis when administered as a vaccine antigen to late gestation cattle, this study was undertaken to determine if the same vaccine antigen could induce a Type-1-like, in vitro response by T cells from calves that had recovered from C. parvum infection. We isolated peripheral blood mononuclear cells from calves that had been previously infected with C. parvum oocysts and incubated them in the presence or absence of the recombinant C. parvum p23 vaccine antigen. We used flow cytometry to simultaneously detect cells in cell cycle and identify the T cell subset containing cycling cells. We also used flow cytometry to identify interferon-gamma positive cells and 2-dimensional gel electrophoresis to profile proteins made by PBMC stimulated with the recombinant p23 vaccine antigen. The results demonstrated that CD4+ T lymphocytes proliferated and that interferon-gamma was synthesized by a subset of stimulated cells. Two-dimensional gel electrophoresis revealed the presence of several cytoplasmic proteins in a size range of approximately 25-80 kDa that were detected in p23-stimulated, but not in unstimulated, cytoplasmic samples. Together, the results show that the recombinant p23 vaccine antigen can stimulate a Type-1-like immune response by T cells from calves that have recovered from C. parvum infection.

Possible mechanisms underlying age-related resistance to bovine babesiosis

Calves infected with the tick-borne parasites Babesia spp. do not develop severe clinical babesiosis. Instead they display persistent low parasitaemias without any apparent ill-effects. This age-related resistance not only benefits the host, but also furthers parasite transmission. Both calves and adult animals respond with a Th I immune response to primary infection. Here we hypothesize that the difference in the outcome of infection may at least partly be explained by the localization and timing of the inflammatory response: in calves NO production occurs early and appears to be concentrated in the spleen. On the other hand, there is evidence that a delayed and systemic inflammatory response occurs in adult animals that is ineffectual and probably contributes to the pathogenesis. An improved understanding of the possible mechanisms that underlie this phenomenon may lead to new approaches for the treatment and immune prophylaxis of the disease.

Immunophenotypical Characterization of Lymphocyte Subpopulations of the Uterus of Non-pregnant and Pregnant Goats

The increased susceptibility during pregnancy to certain pathogens that cause abortions may be related to changes in the distribution and phenotype of lymphocyte subpopulations in the uterus. Histological, electron microscopic and immunocytochemical techniques were used in this study to examine whether such variations occur in different stages of the reproductive cycle of goats. The study of non-pregnant goats showed that most uterine lymphocytes were T cells and displayed both an intraepithelial and stromal distribution. CD8+ T lymphocytes were more numerous than CD4+ T lymphocytes. In the endometrial epithelium two lymphocyte subpopulations were observed: non-granulated CD2+ CD8+ T lymphocytes and granulated CD2+ CD8- T lymphocytes. During gestation, no lymphocytes were observed in the placentomal area, while a decreased number of T lymphocyte subpopulations were found in the inter-placentomal area. In the inter-caruncular epithelium, non-granulated CD2+ CD8+ T lymphocytes disappeared, whereas the granulated CD2+ CD8- T lymphocyte subpopulations increased their number and changed their morphology.

Characterization of the T-cell receptor gamma locus and analysis of the variable gene segment expression in rabbit

The genomic organization and expression of genes of the T-cell receptor gamma (TRG) locus are described for mice and humans, but not for species such as rabbits (Oryctolagus cuniculus), in which gammadelta T cells compose a sizeable proportion of T cells in the periphery. We cloned 200 kb of the rabbit TRG locus and determined the TRGV gene usage in adult and newborn rabbits by RT-PCR. We identified two TRGJ genes, one TRGC gene, and 22 TRGV genes, all of which encoded functional variable regions. One TRGV gene is the unique member of the TRGV2 subgroup, whereas the other genes belong to the TRGV1 subgroup. Evolutionary analyses of TRGV1 genes identified three distinct groups that can be explained by separate duplication events in the rabbit genome. Evidence of gene conversion between TRGV1.1 and TRGV1.6 was observed. Both TRGV1 and TRGV2 subgroup genes were expressed in the spleen, intestine, and appendix of adult rabbits, and the repertoire of TRGV genes expressed in these tissues was similar. In these tissues from newborns, and in skin from adults, only the genes from the TRGV1 subgroup were expressed. Greater TRGV-J junctional diversity was found in tissues from adult compared to newborn rabbits. Our analyses indicate rabbits have a larger germ line encoded TRG repertoire compared with that of mice and humans. In addition, we found TRGV gene usage is alike in most tissues of rabbits similar to that found in humans but in contrast to that found in mice.

In vivo immunomodulatory effects of ixodid ticks on ovine circulating T- and B-lymphocytes

Selected aspects of the ovine immune system were examined during the course of repeated infestations with the ixodid ticks, Haemaphysalis bispinosa and Hyalomma anatolicum anatolicum that naturally infest sheep, either individually or together. By the use of flow cytometry it was shown that total T-lymphocyte numbers were significantly reduced from the sixth through the ninth days of all infestations. Gamma/delta (gammadelta+) and CD8+ T-lymphocytes were significantly depleted during tick feeding in all infested groups. CD4+ T-lymphocyte levels were significantly increased during secondary H. bispinosa and mixed species infestations. Hyalomma anatolicum anatolicum caused a significant increase in circulating B-lymphocytes over several days in both initial and secondary infestations. All infested sheep had increased CD4/CD8 and decreased T/B lymphocyte ratios during exposure to both ticks. Bromodeoxyuridine (BrdU) ELISA was used to measure in vitro proliferation of peripheral blood mononuclear cells stimulated with the T-lymphocyte mitogen Concanavalin A (Con A) after their collection from infested sheep. Significant suppression of in vitro proliferation occurred during first and secondary infestations with H. bispinosa, H. a. anatolicum and with both tick species together, beginning on the sixth day of infestation in all cases. These important tick species of sheep significantly modulate the numbers of immune effector cells and proliferation of T-lymphocytes derived from infested animals.

Targeting T cell responses by selective chemokine receptor expression

Immune responses require the orchestrated migration of T cells throughout the body. Conventional CD4+ and CD8+ alphabeta T cells undergo clonal expansion in the secondary lymphoid tissues, during which they are programmed to migrate into specific non-lymphoid tissues and other lymphoid effector sites such as B cell follicles. By contrast, T cell populations expressing receptors with limited diversity (i.e. gammadelta T cells and NK T cells) appear to be preprogrammed to localize in non-lymphoid tissues where they monitor tissue integrity or serve regulatory functions. By promoting chemotaxis and integrin activation, chemokines and their receptors (in conjunction with surface adhesion molecules) control these T cell homing events. Thus, expression of chemokine receptors defines T cells with tropism for particular tissues and/or microenvironments, and identifies T cell subsets with distinct functional properties.

Composition and Functional Capacity of Blood Mononuclear Leukocyte Populations from Neonatal Calves on Standard and Intensified Milk Replacer Diets

Effects of increased dietary energy and protein on the composition and functional capacities of blood mononuclear leukocyte populations from milk replacer-fed calves were investigated. Holstein bull calves (average age: 4.2 d; n = 19) were assigned randomly to one of two treatment groups. Treatment 1 calves (n = 9) were fed a 20% crude protein, 20% fat milk replacer at a rate of 1.4% body weight of dry matter/d for 8 wk, whereas treatment 2 calves (n = 10) were fed a 30% crude protein, 20% fat milk replacer at a rate of 2.5% body weight of dry matter per day. Composition and functional capacities of mononuclear leukocyte populations from blood samples collected at 4, 18, 32, 46, and 60 d of age were characterized by flow cytometry and ex vivo cell function assays. From 11 to 60 d of age, the mean daily weight gain of treatment 2 calves (1.20 kg/d) was greater than daily weight gain of treatment 1 calves (0.55 kg/d). At 60 d of age, the mean body weight of treatment two calves was 53% (39 kg) greater than the mean body weight of treatment 1 calves. Total numbers of blood leukocytes and the composition of the mononuclear leukocyte population were unaffected by the plane of nutrition. Mitogen-induced DNA-synthesis and immunoglobulin M secretion also were unaffected by dietary treatment. Blood mononuclear leukocytes from calves on intensified diets, however, produced less interferon-gamma and more inducible nitric oxide, suggesting that increased dietary energy and protein affects specific aspects of leukocyte function associated with cell-mediated immunity. The impact of altered interferon-gamma and NO production on the calf s susceptibility to infectious disease are not known. Mononuclear leukocyte populations from all calves also demonstrated age-related changes in composition and functional capacity, likely reflecting natural exposure to infectious agents and maturation of the calfs immune system.

Changes in peripheral leukocyte populations of weak calf syndrome of Japanese black calves

To clarify the cellular immune condition in Japanese Black (JB) calves with a weak syndrome, peripheral leukocyte populations were analyzed by flow cytometry. Twenty JB calves were divided into two groups based on clinical observations; one group of calves was weak, because they had experienced an onset of diarrhea within 3 days after birth and needed treatment (Group 1 ;n=10), and the other group of calves was healthy (Group 2; n=10). With regard to leukocyte populations, CD8(+) cells and gamma delta T cells in Group 1 were markedly lower than those in Group 2 during the experimental periods. It is possible that immune-insufficiency might be based on T lymphocyte function in weak syndrome JB calves during the growth process.

The innate immune response in calves to Boophilus microplus tick transmitted Babesia bovis involves type-1 cytokine induction and NK-like cells in the spleen

The innate immune response to Babesia bovis infection in cattle is age-related, spleen-dependent and, in stabilate inoculated calves, has type-1 characteristics, including the early induction of IL-12 and IFN-gamma. In this study with three calves, parameters of innate immunity were followed for 2 weeks after tick transmission of B. bovis. Each calf survived the acute disease episode without drug intervention, and responded with increased levels of plasma interferon-gamma and type-1 cytokine expression, monocyte/macrophage activation, and CD8+ cellular proliferation in the spleen. The proliferating CD8+ population consisted primarily of NK-like cells, and the expansion occurred in parallel with an increase in IL-15 mRNA expression in the spleen.

Evaluation of type 1 immune response in naïve and vaccinated animals following challenge with Leptospira borgpetersenii serovar Hardjo: involvement of WC1(+) gammadelta and CD4 T cells

Organisms within the Hardjo serovar of Leptospira species are harbored in cattle throughout the world, causing abortion in pregnant animals as well as being shed in the urine, thereby providing sources of zoonotic infection for humans. We recently showed that sterile immunity in vaccinated cattle is associated with induction of a type 1 (Th1) cell-mediated immune response. Here naïve and previously vaccinated pregnant cattle were challenged with a virulent strain of serovar Hardjo and subsequently evaluated for expression of a type 1 immune response. Lymphocytes that responded in a recall response to antigen by undergoing blast transformation were evident in cultures of peripheral blood mononuclear cells (PBMC) from vaccinated cattle throughout the postchallenge test period while those from naïve cattle were evident at one time point only. Nevertheless, beginning at 2 weeks after challenge, gamma interferon (IFN-gamma) was measured in supernatants of antigen-stimulated PBMC cultures from nonvaccinated animals although the amount produced was always less than that in cultures of PBMC from vaccinated animals. IFN-gamma(+) cells were also evident in antigen-stimulated cultures of PBMC from vaccinated but not from nonvaccinated animals throughout the postchallenge period. The IFN-gamma(+) cells included CD4(+) and WC1(+) gammadelta T cells, and a similar proportion of these two subpopulations were found among the dividing cells in antigen-stimulated cultures as ascertained by carboxyfluorescein succinimidyl ester loading. Finally, while naïve and vaccinated animals had similar levels of antigen-specific immunoglobulin G1 (IgG1) following challenge, vaccinated animals had twofold-more IgG2. In conclusion, while infection may induce a type 1 response we suggest that it is too weak to prevent establishment of chronic infection.

Bovine gamma delta T cell subsets express distinct patterns of chemokine responsiveness and adhesion molecules: a mechanism for tissue-specific gamma delta T cell subset accumulation

Subsets of gammadelta T cells localize to distinct tissue sites in the absence of exogenous Ag stimulation or development of effector/memory cells. Selective lymphocyte homing from the blood into tissues is controlled by a multistep process involving vascular and lymphocyte adhesion molecules, and G protein-linked chemokine receptors. The role of these mechanisms in the tissue tropism of gammadelta T cells is still poorly understood. In this study, we demonstrate that a subset of gammadelta T cells, most of which express an antigenically distinct TCR and are characterized by coexpression of CD8, selectively accumulated in tissues that expressed high levels of the mucosal vascular addressin, mucosal addressin cell adhesion molecule 1. These cells expressed higher levels of alpha(4)beta(7) integrins than other gammadelta T cell subsets and selectively migrated to the CCR7 ligand secondary lymphoid-tissue chemokine (CCL21). Integrin activation by CCL21 selectively increased CD8(+)gammadelta T cell binding to recombinant mucosal addressin cell adhesion molecule 1. These results suggest that the tropism of circulating CD8(+)gammadelta T cells for mucosal tissues is due, at least in part, to selective developmental expression of adhesion molecules and chemokine receptors.

The WC1(+) gammadelta T-cell population in cattle: a possible role in resistance to intracellular infection

Intracellular infections are important in veterinary medicine and detailed understanding of the associated immune responses is needed for optimal development of strategies based on diagnosis and vaccination. It is generally accepted that cell-mediated immune responses are of greatest importance in intracellular infections and recent studies from several bovine models of infection indicate that WC1(+) gammadelta T-cells have a number of possible levels of involvement, which remain incompletely defined. Investigations of experimental infection with Mycobacterium bovis in cattle have indicated that WC1(+) gammadelta T-cells are among the first cells to accumulate at initial sites of infection, an observation which has been linked with decreased numbers of these cells in the circulation within days of infection. These WC1(+) gammadelta T-cells have been shown to respond in vitro, both to protein antigens and to non-protein, phosphate containing antigens of M. bovis and to be capable of producing IFN-gamma. Studies of M. bovis infection in calves depleted of WC1(+) gammadelta T-cells by monoclonal antibody have suggested that the presence of these cells is associated with development of a Th1-biased acquired immune response. In combination, these observations allow speculation regarding a possible role for WC1(+) gammadelta T-cells as a link between the innate and acquired immune systems which is instrumental in establishing an appropriate response.

Age-related innate immune response in calves to Babesia bovis involves IL-12 induction and IL-10 modulation

There is a strong innate immunity in calves to infection with Babesia bovis. Interleukin (IL)-12 and IL-10 have been shown in vitro to be important immunoregulatory cytokines. Here we demonstrate in vivo that the protective innate response in young calves to infection with virulent B. bovis involves the early appearance of IL-12 and interferon-gamma (IFN-gamma) transcripts in the spleen. In contrast, IL-12 and IFN-gamma mRNA expression in the spleens of adult cattle that succumbed to the infection was delayed and depressed and occurred within the context of IL-10 expression. Also in contrast with calves, there was no detectable antibody response before death in adults. A vigorous CD8+ T-cell expansion occurred in the spleens of both calves and adults.

Modulation of immune responses to Mycobacterium bovis in cattle depleted of WC1(+) gamma delta T cells

It is accepted that cell-mediated immune responses predominate in mycobacterial infections. Many studies have shown that CD4(+) T cells produce Th1 cytokines, such as gamma interferon (IFN-gamma), in response to mycobacterial antigens and that the cytolytic activity of CD8(+) cells toward infected macrophages is important. However, the extent and manner in which gamma delta T cells participate in this response remain unclear. In ruminants, gamma delta T cells comprise a major proportion of the peripheral blood mononuclear cell population. We have previously shown that WC1(+) gamma delta T cells are involved early in Mycobacterium bovis infection of cattle, but their specific functions are not well understood. Here we describe an in vivo model of bovine tuberculosis in which the WC1(+) gamma delta T cells were depleted from the peripheral circulation and respiratory tract, by infusion of WC1(+)-specific monoclonal antibody, prior to infection. While no effects on disease pathology were observed in this experiment, results indicate that WC1(+) gamma delta T cells, which become significantly activated (CD25(+)) in the circulation of control calves from 21 days postinfection, may play a role in modulating the developing immune response to M. bovis. WC1(+)-depleted animals exhibited decreased antigen-specific lymphocyte proliferative response, an increased antigen-specific production of interleukin-4, and a lack of specific immunoglobulin G2 antibody. This suggests that WC1(+) gamma delta TCR(+) cells contribute, either directly or indirectly, toward the Th1 bias of the immune response in bovine tuberculosis--a hypothesis supported by the decreased innate production of IFN-gamma, which was observed in WC1(+)-depleted calves.

Characterization of host responder types after a single Cooperia oncophora infection: kinetics of the systemic immune response

After primary infection with 100,000 third stage larvae of the intestinal nematode Cooperia oncophora in 3-month-old calves, a high variability in egg output and worm counts is observed. Based on this variability, infected animals can be divided in different responder types. The three major phenotypes can be classified as high, intermediate and low responder animals. We investigated whether calves classified into different responder types show different immune responses during infection. Peripheral blood eosinophil counts and flow cytometric analysis of different lymphocyte subsets of the blood did not reveal major differences between infected and control animals, nor between responder types. However, the levels of Cooperia-specific immunoglobulin (Ig)G1 and IgA during primary infection were significantly higher in intermediate responders than in low responders. In the intermediate responders, isotype specific responses were negatively correlated with parasitological parameters expressing worm expulsion and influence on worm fecundity. Total serum IgE levels were elevated in most of the infected animals. A quantitative positive relationship between worm counts and total serum IgE levels was observed. Based on the observed correlations, we propose a role for the humoral response against the maintenance of the infection in the gut.

Molecular approaches to elucidating innate and acquired immune responses to Babesia bovis, a protozoan parasite that causes persistent infection

For many vector-transmitted protozoal parasites, immunological control of acute infection leads to a state of persistent infection during which parasitemias may cycle unnoticed in infected but otherwise clinically healthy animals. Achieving persistent infection is a strategy that favors parasitism, since both host and, therefore, parasite survive, and endemically infected animal populations provide a reservoir of parasites continually available for subsequent transmission. Examples of the major economically important protozoan pathogens that cause persistent infection in mammals include the related Theileria and Babesia parasites as well as Trypanosoma species. Control of acute infection and maintenance of clinical immunity against subsequent infection are determined by the interplay of innate and acquired immune responses. This review will focus on approaches taken to gain an understanding of the molecular basis for innate and acquired immunity against the hemoprotozoan parasite of cattle, Babesia bovis. Knowledge of mechanisms used by the parasite to survive within infected cattle from acute to persistent infection combined with definition of the correlates of protective immunity in cattle should be applicable to designing effective vaccines.

Bovine neonatal immunology

The majority of early, in utero immune development occurs independent of antigen exposure. Only later during development can a fetus respond to antigens, and even then the response depends on the stage of fetal development and the nature of the antigen. At birth, the neonate is rapidly exposed to large numbers of potential pathogens. Although immunocompetent, the neonate is immunonaive and dependent on passively acquired maternal immunoglobulins, immune cells, and other substances from colostrum for protection. Neonates that suffer failure of passive transfer of maternal immunoglobulins may be at increased risk for disease; however, many other factors interact in conjunction with the level of passively acquired immunoglobulin to determine the occurrence of disease. These include, but are not limited to, management, environment, hygiene, infection pressure, virulence of organisms, and antibody specificity. In addition to immunoglobulins, colostrum contains large numbers of immune cells and cytokines. It is thought that the primary role for the cellular component of colostrum is to interact with the development of local immunity and to modulate active immunization of the neonatal intestine. In particular, T lymphocytes are thought to transfer immune functions and secrete cytokines. Although most of the major cytokines have been identified in colostrum and milk, their biologic effects on the neonate have yet to be determined.

Antigen recognition and immunomodulation by gamma delta T cells in bovine tuberculosis

This report describes the in vitro proliferative responses of peripheral blood gammadelta T cells to defined mycobacterial protein Ags and the immunomodulatory effect of gammadelta T cells in cattle infected with Mycobacterium bovis. gammadelta T cell responses were specific to M. bovis infection because they were detected in cattle either experimentally or naturally infected with M. bovis, but were not present in uninfected controls. Proliferating gammadelta T cell cultures produced enhanced levels of IFN-gamma and TGF-beta, but not IL-2 in response to the more immunodominant mycobacterial AGS: Depletion of gammadelta T cells from PBMC resulted in an increased Ag-specific proliferation in half the animals tested, indicating a suppressive effect of gammadelta T cells upon other (alphabeta) T cell responses. Because gammadelta T cells constitute a major T cell population in the peripheral blood of cattle, the activities of gammadelta T cells described in this report could make a significant contribution to the immune response in bovine tuberculosis.

Regression of canine oral papillomas is associated with infiltration of CD4+ and CD8+ lymphocytes

Canine oral papillomavirus (COPV) infection is used in vaccine development against mucosal papillomaviruses. The predictable, spontaneous regression of the papillomas makes this an attractive system for analysis of cellular immunity. Immunohistochemical analysis of the timing and phenotype of immune cell infiltration revealed a marked influx of leukocytes during wart regression, including abundant CD4+ and CD8+ cells, with CD4+ cells being most numerous. Comparison of these findings, and those of immunohistochemistry using TCRalphabeta-, TCRgammadelta-, CD1a-, CD1c-, CD11a-, CD11b-, CD11c-, CD18-, CD21-, and CD49d-specific monoclonal antibodies, with previously published work in the human, ox, and rabbit models revealed important differences between these systems. Unlike bovine papillomavirus lesions, those of COPV do not have a significant gamma/delta T-cell infiltrate. Furthermore, COPV lesions had numerous CD4+ cells, unlike cottontail rabbit papillomavirus lesions. The lymphocyte infiltrate in the dog resembled that in human papillomavirus lesions, indicating that COPV is an appropriate model for human papillomavirus immunity.

Lymphocyte subset proliferative responses of Mycobacterium bovis-infected cattle to purified protein derivative

Despite highly successful eradication efforts in several countries, Mycobacterium bovis infection of cattle remains a significant health concern worldwide. Immune mechanisms of resistance to and/or clearance of M. bovis infection of cattle, however, are unclear. Recent studies have provided evidence supporting a role for CD4(+), CD8(+), and gammadelta TCR(+) T cells in the response of cattle to M. bovis. In the present study, we utilized a flow cytometric-based proliferation assay to determine the relative contribution of individual lymphocyte subsets in the response to M. bovis infection and/or sensitization with mycobacterial purified protein derivative (PPD). Peripheral blood mononuclear cells (PBMC) from M. bovis-infected cattle proliferated in response to in vitro stimulation with M. bovis PPD. CD4(+) T cells and gammadelta TCR(+) cells were the predominate subsets of lymphocytes responding to PPD. gammadelta TCR(+) cells also proliferated in non-stimulated cultures; however, the gammadelta TCR(+) cell proliferative response of infected cattle was significantly (p<0.05) greater in PPD-stimulated cultures as compared to non-stimulated cultures. Intradermal injection of PPD for comparative cervical testing (CCT) induced a boost in the in vitro proliferative response of CD4(+) but not gammadelta TCR(+) cells of infected cattle. Administration of PPD for CCT also boosted interferon-gamma (IFN-gamma) production by PBMC of infected cattle following in vitro stimulation with M. bovis PPD. Injection of PPD for CCT did not, however, elicit a proliferative or IFN-gamma response in cells isolated from non-infected cattle. These data indicate that CD4(+) and gammadelta TCR(+) cells of M. bovis-infected cattle proliferate in a recall response to M. bovis PPD and that the CD4(+) cell response is boosted by intradermal injection with PPD for CCT.

Immunohistochemical characterization of lung lesions induced experimentally by Mycoplasma agalactiae and Mycoplasma bovis in goats

Goats aged 3 months were inoculated with a recent isolate of Mycoplasma agalactiae (five animals) or Mycoplasma bovis (five animals) by a combined (intratracheal+intranasal) route. Two control goats were inoculated by the same route with sterile mycoplasma broth. Animals were killed 14 or 21 days after infection. At necropsy, tracheal and lung tissue was taken for pathological and immunohistochemical examination to determine changes in the lymphocyte subpopulations in the bronchus-associated lymphoid tissue (BALT). Consolidation of the lungs was not observed in any animal. M. agalactiae or M. bovis was recovered from the respiratory tract and lung of all but two infected animals. Both Mycoplasma spp. induced a moderate bronchointerstitial pneumonia, characterized by lymphoid hyperplasia of the BALT and infiltration of mononuclear cells into the alveolar walls. The predominant phagocytic cell in the pulmonary parenchyma and the airways was the macrophage. The main cellular type in the BALT was the CD3(+)T lymphocyte, and the ratio of CD4(+): CD8(+)cells was >1. It is likely that cellular immune mechanisms, through the activation of CD4(+)T lymphocytes, plays a prominent role in the acute and subacute phase of these infections.

Identification of Ostertagia ostertagi specific cells in bovine abomasal lymph nodes

To investigate the contribution of different bovine cell subpopulations in the development of in vitro induced responses by Ostertagia ostertagi third larval antigen extract (L3), bovine abomasal lymph node cell suspensions were depleted of specific cell populations. The depleted cell suspensions were subsequently assayed for their proliferative responses to O. ostertagi L3 antigen extract. Proliferative responses to O. ostertagi L3 antigen extract were restricted to a CD2+ CD4- CD8- cell population and MHC II+ cells different from B-cells were of major importance. Depletion of CD4, CD8, CD4CD8, IgM or CD21 positive cells did not decrease proliferation to L3 antigen extract. Depletion of gammadelta T-cells, which also comprise a subpopulation of CD2+ CD4- CD8- cells, reduced proliferation to L3 antigen extract only in one animal. The results suggest that either gammadelta T-cells could be involved in the proliferation or that another as yet unidentified population is important for proliferation. The precise role of these populations during infection with O. ostertagi and the mechanism by which these cells may influence the host immune response are important issues that remain to be elucidated.