CSFV免疫状态下外周血T淋巴细胞亚群变化及对PRRS的免疫保护作用

建立CSFV疫苗免疫状态下猪繁殖与呼吸综合征病毒（PRRSV）感染动物模型。测定CSFV抗体阻断率和PRRSV抗体S/P值,根据CSFV抗体阻断率和PRRSV感染情况分析猪外周血中T淋巴细胞亚群变化和对PRRS的保护促进作用。采用流式细胞术（FAC）检测猪外周血T淋巴细胞CD3、CD4、CD8亚群的变化。结果,CSFV...     

Parturition invokes changes in peripheral blood mononuclear cell populations in Holstein dairy cows naturally infected with Mycobacterium avium subsp. paratuberculosis

Johne's disease (JD) is characterized by a protracted period of subclinical infection. Infected cows may remain in the subclinical state until stressors such as parturition and lactation invoke more clinical signs of disease. The objective of this study was to evaluate changes in the percentages of CD4(+), CD8(+), and gammadelta T-cells, B-cells, monocytes, as well as the expression of the activation marker, CD5, on these cell subpopulations in the peripheral blood of dairy cows naturally infected with Mycobacterium avium subsp. paratuberculosis (MAP) during the periparturient period. Peripheral blood mononuclear cells (PBMCs) were collected from 3 wk pre- to 4 wk post-calving and freshly isolated or cultured for 7d. Day 7 cultures were infected with live MAP at a 10:1 MOI (bacteria to adherent PBMC), and cultures were incubated for an additional 24h. Fluorescent antibody labeling of lymphocyte subsets and monocytes was conducted and analyzed with flow cytometry. Freshly isolated PBMCs from subclinical cows expressed a greater (P<0.05) percentage of CD8(+) and gammadelta T-cells compared with clinical cows. The percentage of CD4(+) T-cells increased (P<0.08) in clinical cows as parturition approached. During the postpartum period, clinical cows had greater (P<0.05) CD4:CD8 ratios compared with subclinical and control cows. After 8d, uninfected PBMCs from clinical cows had greater (P<0.05) percentages of CD14(+) cells compared with subclinical cows. When infected with live MAP, there was no effect of infection group or parturition on cell subpopulations. In fresh PBMCs, clinical cows expressed lower percentages of CD4(+)CD5(bright) and CD8(+)CD5(bright) compared with control cows, but greater percentages of CD5(dim) cells for all lymphocyte subsets. These results suggest changes in the percentages of lymphocyte subsets, monocytes, and CD5 markers are modulated by both infection status and the periparturient period.     

猪瘟病毒感染对猪外周血T淋巴细胞亚群及TNF-α和IFN-γ的影响

为探讨猪瘟病毒(CSFV)弱毒株T株和野毒株G株感染对猪外周血T淋巴细胞亚群、TNF-α和IFN-γ的影响,本研究应用流式细胞术和ELISA等方法检测CSFV感染猪与未感染猪的白细胞凋亡、CD4+与CD8+T淋巴细胞亚群数量的动态变化以及TNF-α和IFN-γ的动态变化。结果表明,猪感染CSFV T株和G株第4 d和第7 d后CD4+T淋巴细胞比例分别为28.6%、26%和26%、20%,未感染前分别为33.4%和36.8%。猪感染CSFV T株和G株第4 d、第7 d后CD8+T淋巴细胞比例分别为41%、32%和38%、25%,感染前分别为43.8%和48.8%。外周血白细胞凋亡的检测结果显示,猪感染CSFV T株和G株第7 d后,白细胞凋亡比例分别为8.35%和9.89%,未感染的猪为1.63%。ELISA检测结果表明,猪感染CSFV T株和G株第7 d后,TNF-α的产生量分别为553.4 pg/mL和594.2 pg/mL;IFN-γ的产生量分别为8.2 pg/mL和9.8 pg/mL,未感染猪分别为498 pg/mL、12.5 pg/mL。以上结果提示,CSFV感染会引起机体免疫相关细胞及免疫分子发...     

Flow cytometric immune profiling of specific-pathogen-free chickens before and after infectious challenges

Broilers and layer chickens have been intensively selected for production parameters. This selection has affected immune capacity. Consequently, the fine-tuning of immune responses is becoming important for maximum productivity. Flow cytometry is a recurrent technology used for the immunophenotyping of birds. Studies, however, have focused on the mechanism of specific diseases or have used animals whose immunological condition could be biased-by vaccination or environmental stressors, for example. The aim of this study was to evaluate the immune status of specific-pathogen-free birds across different age ranges to characterize the natural changes that occur over time. Additionally, specific-pathogen-free chickens were challenged with four infectious agents, allowing identification of the subpopulations of peripheral blood immune cells that are consistently altered under various conditions. Several lymphocyte subsets vary naturally with aging, so the interpretation of results using animals of different age ranges must proceed with care. Parameters such as CD8(+)CD28(-), CD8αα(+), CD4(+)CD8(+), and CD8(+)TCRVβ1(+) have been shown to be valuable in understanding immune changes during disease. The use of these data allows a determination of the consistency of cytometric parameters under various conditions, which should ease the interpretation of immunophenotyping and the future application of cytometric analysis in the poultry industry.     

Lymphocyte subpopulations in peripheral blood of lambs experimentally infected with bovine respiratory syncytial virus

The lymphocyte subpopulations of peripheral blood of normal lambs and lambs experimentally infected with bovine respiratory syncytial virus (RSV) were analysed by flow cytometry, using a panel of monoclonal antibodies against specific lymphocyte epitopes. Experimental infection with bovine RSV was characterized by a significant rise in SBU-T8+ (CD8+ or cytotoxic) T cells and a significant reduction in SBU-T4+ (CD4+ or helper) T cells and B (LCA p220+) lymphocytes (P less than 0.05). The helper/suppressor (CD4/CD8) ratio was reduced from 3.91 on the day of experimental infection to 1.13 on 10 days after experimental infection (P less than 0.001). The total number of SBU-T4+ (CD4+) and B cells returned to pre-inoculation values 14 days after experimental infection but the helper/suppressor ratio remained depressed up to 21 days post-inoculation.     

猪免疫口蹄疫灭活疫苗后外周血细胞亚群变化的研究

为了解动物在注射口蹄疫灭活疫苗后机体免疫状态的变化情况,本试验选取口蹄疫阴性猪进行疫苗免疫,利用流式细胞术检测方法来测定外周血中免疫细胞亚群的变化。结果显示,树突状细胞在免疫后能够快速产生应答,在24 h内其含量出现明显升高;B细胞和Th细胞含量在免疫后第10天都出现了明显升高,且两者之间的变化存在着高度相关性,而抗体水平的变化情况与这两种细胞含量的变化也高度相关;细胞毒性T细胞含量在免疫后也出现升高。这说明灭活疫苗免疫动物后不仅可以激活机体的体液免疫应答反应,产生高水平抗体,发挥免疫保护作用,更重要的是能够在短时间内激发机体的固有免疫应答,同时能够激活细胞免疫应答,全方位发挥免疫保护作用。     

The effects of prednisone and azathioprine on circulating immunoglobulin levels and lymphocyte subpopulations in normal dogs.

This study investigates serum immunoglobulin (SIg) levels and lymphocyte subpopulations in normal dogs in response to putative immunosuppressive doses of prednisone and/or azathioprine. The objectives were to quantify SIg levels and lymphocyte subpopulations, including Thy-1+, CD4+, CD8+ and B cells, in normal dogs both before and after the administration of prednisone and/or azathioprine at 2 mg/kg, PO, each. Eighteen beagles were divided into 3 groups of 6 dogs each. Blood samples for radial immunodiffusion assay of IgG, IgM and IgA, complete blood count (CBC)and flow cytometry were collected prior to the administration of any drugs and again after 14 d of azathioprine, prednisone or azathioprine and prednisone. Peripheral blood mononuclear cells were isolated using density centrifugation and were incubated with monoclonal antibodies reacting with CD4+, CD8+, Thy-1+ and membrane immunoglobulin. Lymphocyte subsets were quantified using flow cytometry. Azathioprine-treated dogs had no significant changes in SIg levels or lymphocyte subpopulations. Prednisone-treated dogs had significant (P < 0.05) decreases in all SIg levels, all lymphocyte subpopulations and erythrocyte numbers, and had an increase in neutrophil counts. Prednisone and azathioprine-treated dogs had significant (P < 0.05) decreases in serum IgG levels and Thy-1+ and CD8+ lymphocyte subpopulations, with an increase in the CD4:CD8. These dogs also had a significant decrease in erythrocyte number and a significant increase in the monocyte count. These findings suggest that azathioprine and prednisone in combination or prednisone alone may be useful for the treatment of T cell-mediated diseases since decreased circulating T cell levels were demonstrated following treatment. The combination of drugs or azathioprine alone may not be appropriate for treatment of acute or autoantibody-mediated immune disease, because SIg levels were minimally affected by treatment.     

Characterization of monoclonal antibodies to equine interleukin-10 and detection of T regulatory 1 cells in horses

Interleukin-10 (IL-10) terminates inflammatory immune responses and inhibits activation and effector functions of T-cells, monocytes, macrophages and dendritic cells. IL-10 has also been found to be a key cytokine expressed by subpopulations of regulatory T-cells. In this report, we describe the generation and characterization of three monoclonal antibodies (mAbs) to equine IL-10. The antibodies were found to be specific for equine IL-10 using different recombinant equine cytokine/IgG fusion proteins. Two of the anti-equine IL-10 mAbs were selected for ELISA to detect secreted IL-10 in supernatants of mitogen stimulated equine peripheral blood mononuclear cells (PBMC). The sensitivity of the ELISA for detecting secreted IL-10 was found to be around 200pg/ml. The production of intracellular IL-10 was measured in equine PBMC by flow cytometry. PBMC were stimulated with phorbol 12-myristate 13-acetate (PMA) and ionomycin in the presence of the secretion blocker Brefeldin A. All three anti-IL-10 mAbs detected a positive population in PMA stimulated lymphocytes which was absent in the medium controls. Around 80% of the IL-10(+) cells were CD4(+). Another 15% were CD8(+) cells. Double staining with IL-4 or interferon-gamma (IFN-gamma) indicated that PMA and ionomycin stimulation induced 80% IL-10(+)/IFN-gamma(+) lymphocytes, while only 5% IL-10(+)/IL-4(+) cells were observed. By calculation, at least 60% of the IL-10(+)/IFN-gamma(+) cells were CD4(+) lymphocytes. This expression profile corresponds to the recently described T regulatory 1 (T(R)1) cell phenotype. In summary, the new mAbs to equine IL-10 detected native equine IL-10 by ELISA and flow cytometry and can be used for further characterization of this important regulatory cytokine in horses.     

Study on Change of Lymphocyte Subpopulations in Swine after Immunization with Inactivated Vaccine

In order to investigate the change of lymphocyte subpopulations in the peripheral blood after injection of foot-and-mouth disease inactivated vaccine, blood samples were collected after vaccination and assayed with flow cytometry.The results showed that the proportion of dendritic cells (DCs) increased significantly in a short time after vaccination, after which the number of B cells, helper T (Th) cells and cytotoxic lymphocytes (CTLs) also raised with significance.The changing trend of B cells proportion was highly relevant with Th cells and a same correlation also appeared in the antibody titers.This indicated that the innate immune system could be activated rapidly after vaccination with the inactivated vaccine, and the adaptive immune response including cellular immune response and humoral immune response could be invoked subsequently.The high level of neutralizing antibodies was of great importance in evading FMDV and the cellular immune response also exerted a crucial role.     

Subpopulations of equine blood lymphocytes expressing regulatory T cell markers

Several distinct T lymphocyte subpopulations with immunoregulatory activity have been described in a number of mammalian species. This study performed a phenotypic analysis of cells expressing regulatory T cell (Treg) markers in the peripheral blood of a cohort of 18 horses aged 6 months to 23 years, using antibodies to both intracellular and cell surface markers, including Forkhead box P3 (FOXP3), CD4, CD8, CD25, interferon gamma (IFNγ) and interleukin 10 (IL-10). In peripheral blood, a mean of 2.2 ± 0.2% CD4+ and 0.5 ± 0.1% CD8+ lymphocytes expressed FOXP3. The mean percentage of CD4+FOXP3+ cells was found to be significantly decreased in horses 15 years and older (1.5%) as compared to horses 6 years and younger (2.7%), but did not differ between females and males and ponies and horses. Activation of peripheral blood mononuclear cells by pokeweed mitogen resulted in induction of CD25 and FOXP3 expression by CD4+ cells, with peak expression noted after 48 and 72 h in culture respectively. Activated CD4+FOXP3+ cells expressed IFNγ (35% of FOXP3+ cells) or IL-10 (9% FOXP3+ cells). Cell sorting was performed to determine FOXP3 expression by CD4(+)CD25(-), CD4(+)CD25(dim) and CD4(+)CD25(high) subpopulations. Immediately following sorting, the percentage of CD4+FOXP3+ cells was higher within the CD4(+)CD25(high) population (22.7-26.3%) compared with the CD4(+)CD25(dim) (17% cells) but was similar within the CD4(+)CD25(dim) and CD4(+)CD25(high) cells after resting in IL-2 (9-14%). Fewer than 2% of cells in the CD4(+)CD25(-) population expressed FOXP3. These results demonstrate heterogeneity in equine lymphocyte subsets that express molecules associated with regulatory T cells. CD4+FOXP3+ cells are likely to represent natural Tregs, with CD4+FOXP3+IL-10+ cells representing either activated natural Tregs or inducible Tregs, and CD4+FOXP3+IFNγ+ cells likely to represent activated Th1 cells.     

Distribution of lymphocyte subsets in the small intestine lymphoid tissue of 1-month-old lambs

Distribution of lymphocyte subpopulations along the small intestine lymphoid tissue has been examined in 1-month-old lambs using flow cytometric and immunohistochemical techniques. Monoclonal antibodies against CD4, CD8, gamma delta, CD45R and B receptors have been employed in samples from continuous ileal Peyer's patch (IPP), discrete jejunal Peyer's patches (JPP), ileocaecal valve lymphoid tissue (ICVPP), mesenteric lymph node (MLN) and intra-epithelial (IEL) and lamina propria (LPL) lymphocytes. Histological studies were also done. Differences in the lymphocyte distribution have been observed between some of the regions examined, especially between IPP and JPP for most of the markers. A remarkable feature was the existence of morphological and lymphocyte distribution differences between ICVPP and IPP, locations that had been traditionally considered as similar. The antibody against CD45R receptor used in this study, that was supposed to mark B cells and some T cells, detected cell populations located in the dome of the follicles in all the samples, whereas the centre was negative. Lymphocytes positive to the B marker employed were located mainly in the centre, suggesting that both antibodies would mark B cells in different maturation status.     

Identification of monoclonal antibodies for immunohistochemical staining of feline B lymphocytes in frozen and formalin-fixed paraffin-embedded tissues.

Commercially-available monoclonal antibodies to B lymphocytes were evaluated for immunohistochemical staining of feline B lymphocytes in frozen and formalin-fixed, paraffin-embedded tissues using an avidin biotin complex immunoperoxidase immunohistochemical technique. Three monoclonal antibodies: F46A and F72A raised to "carnivore" B lymphocytes and RA3.6B2 raised to murine B lymphocytes, stained B lymphocyte-dependent areas of frozen feline lymphoid tissue. In addition, antibody RA3.6B2 stained B lymphocyte dependent areas in formalin-fixed, paraffin-embedded feline tissues. There was no staining of T lymphocyte-dependent areas in either frozen or formalin-fixed tissues. Dual parameter flow cytometry, using an anti-pan-T lymphocyte antibody, revealed that greater than 99% of the cells stained by RA3.6B2 are a population distinct from T lymphocytes. F46A was shown to stain a sub-population of those cells stained with RA3.6B2. These antibodies may be useful in the identification of feline B lymphocytes using immunohistochemistry and flow cytometry and thereby provide additional tools to study B lymphocyte ontogeny and the significance of lymphocyte phenotype in lymphoid neoplasia in cats.     

Lymphocyte subpopulations in peripheral blood of sheep experimentally infected with tick-borne fever.

The lymphocyte subpopulations in the peripheral blood of normal sheep and sheep experimentally infected with Cytoecetes phagocytophila, the causative agent of tick-borne fever, were analysed by flow cytometry, using a panel of monoclonal antibodies against specific lymphocyte epitopes. Experimental infection with tick-borne fever was characterised by a significant reduction in the total number of circulating lymphocytes six days after experimental infection (P less than 0.001). This lymphocytopenia was associated with a significant reduction in the number of B (LCAp220+) and T (CD5+) lymphocytes (P less than 0.001) but there was a significant increase in the number of cells which were neither T nor B (CD5-LCAp220-) cells (P less than 0.01). The reduction in the number of T lymphocytes was due to reduced numbers of circulating CD4+ (helper) T cells, CD8+ (cytotoxic/suppressor) T cells and those with the pan T cell marker (CD5+) but without CD4 or CD8 epitopes (CD4-CD8-). All lymphocytes returned to preinoculation levels 13 to 16 days after experimental infection.     

The bovine lymphoid system: II. Derivation and partial characterlzation of monoclonal antibodies against bovine peripheral blood lymphocytes

We have raised monoclonal antibodies to produce reagents specific for bovine lymphocyte subpopulations. Spleen cells from mice immunized with bovine peripheral blood lymphocytes were fused with X63-Ag8 myeloma cells and eleven myeloma-hybrids which secreted antibody specific for bovine lymphocytes were doubly cloned. Five of the hybrids secreted antibodies which bound to the majority of bovine lymphocytes. Two of these antibodies were specific for polymorphic antigens. One antibody bound to B lymphocytes and serum IgM molecules. The remaining five bound to subpopulations of lymphocytes. Four monoclonal antibodies bound only to bovine cells while six also bound to lymphocytes from other bovidae. None bound to human lymphocytes. We discuss the difficulty of correlating the specificities of monoclonal antibodies to functional lymphocyte subpopulations in outbred animals where few other defined markers are available.     

Identification of anti-human CD antibodies reactive with rhesus macaque peripheral blood cells

Three hundred and seventy seven commercially available monoclonal antibodies (mAb) were tested for their cross-reactivity with rhesus macaque (Macaca mulatta) peripheral blood cells. These antibodies were collected by the animal homologue section of the HLDA8 Workshop in order to assign their potential applicability for in vitro assays. Reactivity of each mAb with lymphocyte, monocyte and granulocyte populations obtained from peripheral blood of adult rhesus macaques was evaluated. Single-colour flow cytometry and indirect labeling method was used in first-round screening. Based on their reactivity with rhesus macaque cells 57 positive mAb were selected for second-round testing. Multi-colour flow cytometry and combinations of direct and indirect labeling was used to compare the reactivity of the respective mAb. In addition, reference reagents known to react with rhesus macaque CD3, CD20 and CD56 were used to further characterization of the reactivity of the selected 57 mAb on peripheral blood cells.     

Evolution of the peripheral blood lymphocyte populations in multiparous rabbit does with two reproductive management rhythms

The emergence of epizootic rabbit enteropathy is leading to changes in weaning protocols in commercial rabbitries. Traditional weaning protocols are being replaced with late weaning, beyond 35 days postpartum (dpp). The main objectives of this study were to compare the peripheral blood lymphocyte populations of multiparous rabbit does under two reproductive rhythms (insemination at 11 dpp and weaning at 28 dpp, insemination at 25 dpp and weaning at 42 dpp), and to assess the influence on those of kits. Samples of peripheral blood were taken in 22 adult females and 44 of their kits at different critical times, and several lymphocytic populations were evaluated by flow cytometry. Additionally, the perirenal fat thickness of does was also measured at partum and weaning to observe if body condition correlates with lymphocyte populations. During whole lactation, counts of total, CD5(+), CD4(+) and CD8(+) lymphocytes of females were generally lower with weaning at 42 dpp compared to 28 dpp. Moreover, counts of total, B and CD5(+) lymphocytes in rabbit does weaned at 42 dpp correlated to their body condition (+0.60 to 0.82; P<0.05), contrary to that observed in rabbit does weaned at 28 dpp. Some correlations between lymphocyte counts in both groups of does and weaning rabbits were observed. At weaning, those young rabbits weaned at 42 dpp had a significantly lower number of CD4(+) lymphocytes than those weaned at 28 dpp (P<0.01). In conclusion, the 42 ddp rabbit does presented a lower number of total lymphocytes and lymphocytic subpopulations during lactation and at weaning, as well as lesser capacity of adjustment during the gestation-lactation cycle.     

Peripheral Blood Neutrophil Function and Lymphocyte Subpopulations in Cycling Mares

The purpose of this study was to evaluate different parameters of the immune status in the mare, during the follicular and the luteal phases of the oestrous cycle, in two consecutive years. Functional competence of peripheral blood neutrophils, such as chemotaxis, phagocytosis and oxidative burst was assessed under physiological cyclic conditions (Exp. I). In the second year of this study (Exp. II), besides peripheral blood neutrophil phagocytosis and oxidative burst analysis, circulating lymphocyte subsets were also characterized. The reproductive status in a total of 17 adult cycling mares was evaluated by ultrasonography and further confirmed by plasma progesterone levels. Chemotaxis tests were performed using porous membranes inserted in transwell chambers. Lipopolysaccharide (LPS) from Escherichia coli and N‐formyl‐Met‐Leu‐Phe (fMLP) were used as chemoattractants. Measurement of phagocytosis and oxidative burst in blood neutrophils were assessed by flow cytometry using commercially available kits. Quantification of T‐lymphocyte subsets was assessed by indirect immunofluorescence staining after incubation with monoclonal antibodies specific for CD2, CD3, CD4 and CD8 cell markers by flow cytometry. Natural killer cells and B cells were estimated mathematically. No significant difference was found in migration, phagocytosis and oxidative burst at either phase of the oestrous cycle. Statistical analysis of total white blood cell counts also showed no significant difference between either phase of the oestrous cycle, although there was a tendency for blood neutrophils to increase in number under the progesterone influence (p = 0.09). Lymphocytic subpopulations did not differ throughout the oestrous cycle. Overall, our results suggest that luteal and follicular phases in cycling mares may not influence the immune status of the mare.     

Lymphocyte subpopulations in peripheral blood of lambs experimentally infected with Pasteurella haemolytica.

The lymphocyte subpopulations in peripheral blood obtained from eleven lambs experimentally infected with Pasteurella haemolytica were compared with those obtained from eight control lambs by flow cytometry, using a panel of monoclonal antibodies against specific lymphocyte epitopes. Experimental infection with P. haemolytica was characterized by a transient but significant reduction in SBU-T1+ (CD5+) T cells and SBU-T4+ (CD4+ or helper) T lymphocytes (P less than 0.05) and a significant rise in lymphocytes which did not express the LCA p220 epitope and the pan T cell surface marker (CD5-LCA p220-) ("null"). The reductions in CD5+ and CD4+ lymphocytes occurred 24 h after experimental infection, returning to preinoculation levels 5 days post inoculation (DPI). Five to 9 days after experimental infection, there was a significant increase in the number of lymphocytes, which expresses the pan T cell surface marker (CD5+) but which were CD4-CD8-. Lymphocyte transformation responses to the mitogen phytohaemagglutinin (PHA) were significantly reduced 24 h after experimental infection with P. haemolytica (P less than 0.05).     

Influence of chronic caprine arthritis-encephalitis virus infection on the population of peripheral blood leukocytes

The influence of caprine arthritis-encephalitis (CAE) virus infection on the population of peripheral blood leukocytes in goats was evaluated. For this purpose two groups of adult dairy female goats were formed. The experimental group consisted of 17 goats, which had been naturally infected for many years. The control group comprised 29 non-infected goats, which originated from CAE-free herd. All goats were clinically healthy. Whole blood was collected and tested in hematological analyzer and light microscope to assess the total number of leukocytes and the percentage of four leukocyte populations--neutrophils, eosinophils, monocytes and lymphocytes. Then, flow cytometry with monoclonal antibodies against several surface antigens (namely CD14, CD2, B-B2, CD4, CD8h, TCR-N6, WC1-N2 and WC1-N3) was performed to assess the proportion of lymphocyte subpopulations. Statistically significant differences (alpha < or = 0.01) were observed only in the subpopulations of T lymphocytes--percentage of all subpopulations were significantly higher in the group of seropositive goats. No statistically significant differences were revealed with respect to the total number of blood leukocytes, the average percentage of blood leukocyte populations and proportions of both T and B lymphocytes.     

Effect of CD4+ T lymphocyte depletion on resistance of Gulf Coast Native lambs to Haemonchus contortus infection

It has been reported that CD4(+) T lymphocytes are important in acquired immunity to gastrointestinal nematode infection. Whether these lymphocytes are also involved in the immune response of naturally resistant Gulf Coast Native (GCN) sheep to Haemonchus contortus infection remains to be defined. The objective of this study was to determine the role of CD4(+) T lymphocytes in this resistance. Ten GCN lambs were randomly assigned to a control (n=5) or a treatment (n=5) group. The treatment consisted of a series of IV injections with mouse anti-ovine CD4(+) T lymphocyte monoclonal antibodies for a period of 3 weeks. After the second treatment, all lambs were experimentally infected with 10,000 H. contortus infective larvae by oral inoculation. All lambs were monitored for fecal egg counts, blood packed cell volumes, white blood cell differential counts and serum antibody responses on a weekly basis. Fluorescence-activated cell sorter (FACS) analysis was done biweekly to enumerate CD4(+) T lymphocytes in peripheral blood. Necropsies were performed at the end of the study and 10% of the contents of the gastrointestinal tract were preserved for nematode enumeration and identification. Also at necropsy, mesenteric lymph nodes were extracted and FACS analysis was run on lymphoid cells. Mean fecal egg counts on day 21 and 28 post-infection and nematode counts at necropsy of the treated group were significantly (p<0.05) higher than that of the control group. Percent CD4(+) T lymphocytes in peripheral blood was significantly (p<0.05) lower in the treatment group than in the control group from day 9 to the end of the study. No differences were found in blood packed cell volumes, white blood cell differential counts, antibody titer or lymph node CD4(+) lymphocytes between groups. Lambs depleted of their CD4(+) T lymphocytes were more susceptible to H. contortus infection than undepleted lambs. The results of this study suggest that CD4(+) T lymphocytes are associated with the natural resistance of GCN sheep to H. contortus infection.