Studies on ovine efferent lymph following infection with Toxoplasma gondii

Parasite distribution and associated immune response during the acute phase of Toxoplasma gondii infection in sheep

BACKGROUND

In many countries, Toxoplasma gondii (T. gondii) is a major cause of reproductive disorders and abortions in the sheep industry, and therefore responsible for important financial and economic losses. In addition, undercooked infected lamb is an important risk factor for human toxoplasmosis. In the present study, the initial phase of the infection was investigated: the parasite's entry site, the subsequent distribution of the parasite and the host-immune response.

RESULTS

Parasite DNA was already detected in the cranial small intestinal mucosa the first days after oral infection with T. gondii tissue cysts. Simultaneously, high IFN-gamma and IL-12 responses were induced mainly in the mesenteric lymph nodes. The emergence of IgG1 (at 8dpi), and IgG2 (at 11 dpi) was accompanied by a decrease or even disappearance of the IFN-gamma and IL-12 response in the Peyers' patches (PP), PBMC's and popliteal LN's. Meanwhile the parasite DNA could be recovered from most mucosal and systemic tissues to become undetectable in the small intestine, popliteal LN, PBMC and spleen 3 weeks pi.

CONCLUSIONS

Our results indicate that parasites enter the cranial small intestine the first days after infection and that after an increase the first two weeks after infection, the parasite DNA levels in the intestine drop below the detection limit three weeks after infection. This coincides with an increase in parastic-specific serum IgG1 and IgG2 and a decrease of the antigen-specific IFN-gamma response in PP, PBMC and popliteal LN. We suggest a role for IFN-gamma and IL-12 in controlling the infection.

Development of lesions and tissue distribution of parasite in lambs orally infected with sporulated oocysts of Toxoplasma gondii

The host-pathogen interaction is as a key feature during the formation of tissue cysts of Toxoplasma gondii within intermediate hosts. In this study, we investigated whether oral infection of lambs with T. gondii oocysts may be used as an experimental model in sheep to study this interaction, with the main objective being to detect the presence and distribution of lesions and parasite within different organs at different time points after oral infection. Lambs were infected with 5 × 10(3) and 5 × 10(5) sporulated T. gondii oocysts and culled at 2, 3, 5 and 6 weeks post-infection (WPI). During the infection, rectal temperature of the animals and serological antibodies against T. gondii were monitored. The presence of inflammatory lesions and parasite were evaluated through histological and immunohistochemical methods at different organs (brain, liver, lung, heart and lymph nodes). The lambs showed no clinical signs other than fever, and lesions appeared mainly in the brain, characterized by glial foci and perivascular cuffs, and in the heart, denoted by foci of interstitial myositis. Tissue cysts and tachyzoite-like structures were observed at all time points studied in the brain, where together with the glial foci they appeared mainly in the cerebral cortex of the forebrain and in the midbrain, but also in the heart, lung and lymph nodes. This study shows that oral infection with sporulated oocysts in lambs may provide a model for investigating the host-parasite interaction in situ during the development of tissue cysts.

Analysis of the immune response in sheep efferent lymph during Salmonella abortusovis infection

The efferent lymph duct of the ovine prescapular lymph node was cannulated, and Salmonella abortusovis (SAO), a specific pathogen for sheep inducing abortion and mortality of newborn lambs, was inoculated by the subcutaneous route in this lymph node drained area. While the prescapular lymph node draining the inoculation site represented an efficient barrier for the vaccinal SAO Rv6 strain spreading, SAO 15/5 virulent bacteria were steadily detected in efferent lymph of infected sheep. The inoculation of the virulent strain of SAO induced a greater increase of the cell output than did the attenuated vaccinal strain, but proportions of blast cells appearing in the efferent lymph were similar in both cases. Flow cytometry analysis showed that B and T cell outputs were both increased during SAO infections, but while T cell subset proportions slightly decreased, B cell percentages significantly rose, and, at the peak response, almost all of the lymphoblast cells were activated B cells. Typical antibody profiles characteristic of a primary immune response were observed, and antibody titres were greater in the efferent lymph of animals inoculated with the virulent strain of SAO. Many of the cytokine mRNAs we investigated were steadily detected by RT-PCR in efferent lymph cells of control sheep, but frequencies of detection of IL-2, IFN gamma, IL-1 beta and TNF alpha mRNAs were augmented in efferent lymph cells following inoculation of both SAO virulent or vaccinal strains. IL-10 and IL-8 mRNAs could only be detected after a SAO inoculation, while detection of IL-4 mRNAs was increased only in efferent lymph cells from SAO virulent strain-infected sheep. The efferent lymph cannulation technique thus appeared a very powerful way to study the in vivo development of the immune response to SAO, in its natural host, the sheep.

Effects of Trypanosoma evansi on the output of cells from a lymph node draining the site of Pasteurella haemolytica vaccine administration

The prefemoral efferent lymphatics of sheep infected with Trypanosoma evansi and inoculated with P. haemolytica vaccine and of those given only the vaccine, were surgically cannulated to study the effects of the infection on the total cellular output and output of blast cells from the node in response to the vaccine. T. evansi delayed and depressed the increases in total cell and lymphoblast outputs. In uninfected sheep, the total cellular output increased and peaked at more than twice the prevaccination values on days 4 and 5 after primary vaccination, but the increases were smaller and peaked on days 6 and 8 after primary vaccination in the infected sheep. The output of lymphoblasts mirrored the total cell output, though it was suppressed to a greater degree by T. evansi. The output of blasts peaked at more than 8 and 14 times the prevaccination values in the uninfected animals after primary and secondary (booster) vaccinations, respectively; but in infected animals, it peaked at twice the prevaccination values after the primary vaccination and showed no increase after booster vaccination until 11 days later. It is concluded that the inhibition of total and blast cell outputs by T. evansi may limit the early systemic dissemination of antigen-specific cells, thus playing a role in the induction of immunosuppression by the parasite.

Primary and secondary responses of the ovine lymph node to Toxoplasma gondii: cell output in efferent lymph and parasite detection

Efferent lymphatic cannulation was used to study the dissemination of strain S48 of Toxoplasma gondii and the cell output from the prefemoral lymph node, after infection of both "naive" and vaccinated sheep. In the former the mean cell output decreased for 3 days before reaching a peak at 11 and 12 days, but in vaccinated ewes a similar drop in cell output and subsequent peak occurred significantly earlier, at 24 h and 5 days, respectively. The cellular response in both types of sheep was largely due to a marked increase in blast cells. The detection of live toxoplasms and parasite DNA by mouse inoculation and the polymerase chain reaction, respectively, gave similar results; the parasite was demonstrated in lymph from days 3 to 12 during a primary infection but with a sharp cut-off after day 9 coinciding with the peak blast cell response. Very little evidence of T. gondii was found in lymph of vaccinated sheep after challenge. Immunity, which is thought to be largely T-cell mediated and is sustained without continuous antigenic stimulation, suppresses dissemination of the parasite in the lymph and therefore to other sites, which might include the gravid uterus.

Immune responses of the ovine lymph node to Chlamydia psittaci. A cellular study of popliteal efferent lymph

The popliteal efferent lymphatics were cannulated in sheep of two categories, seronegative or immune to Chlamydia psittaci. Following subcutaneous injection of live C. psittaci or control material into the draining area of the popliteal node, sequential samples of efferent lymph were collected and analysed. Both categories of sheep responded to C. psittaci with increased outputs of lymphocytes and blast cells. Numbers of blast cells rose both absolutely and as a proportion of the total. Plasmablasts increased in number only in seronegative sheep. Outputs of total T cells (CD5+), helper T cells (CD4+), cytotoxic/suppressor T cells (CD8+) and non-helper, non-suppressor T cells (T19) were maximal 4 and 7 days after challenge in immune and seronegative sheep, respectively. Proportionally, CD4+ T cells declined, CD8+ T cells increased and T19 cells were unaltered with time after infection. Chlamydial antigens could not be demonstrated in the cells of efferent lymph by an immunoperoxidase method. The results of this preliminary study show that both T and B cell responses are involved in immunity to C. psittaci.