Immune responses against bovine tropical theileriosis: with particular reference to reimmunisation with Theileria annulata infected cell lines

Abstract

This study describes the effect of allogeneic immune responses on immunisation against Theileria annulata with a parasite infected cell line. The dynamics of lymphocyte populations during development of the immune response against the parasite after cell line immunisation and sporozoite challenge were investigated in peripheral blood, and lymph efferent from the lymph node draining the site of infection.ph node draining the site of infection. T. annulata infected cell lines have been used as vaccines against tropical theileriosis in several countries. Inoculated animals produce a strong response against the allogeneic MHC antigens of the immunising cell line followed by an and parasite response. There is evidence that immunity to the parasite wanes in the absence of challenge and reimmunisation is often recommended. However, the effect of pre-existing allogeneic responses generated after first immunisation with T. annulata infected cell lines upon development of immunity against the parasite at the time of reimmunisation, is not known. To investigate this, an allogeneic response was first generated in the animals followed by immunisation with a T. annulata infected cell line of the same BoLA type. A mild allogeneic response generated by inoculation of uninfected leucocytes did not affect the development of immunity during cell line immunisation. However, a strong anti MHC response generated by skin grafting interfered with the development of a parasite specific immune response when the animals were immunised with lxlO6 cells. The effect of the allogeneic response was more marked when vaccination was carried out with a lower cell dose of 1 xlO4 cells, where the development of immunity against T. annulata was completely blocked. These observations are of immediate importance in endemic areas where T. annulata infected cell lines are being used as vaccines to control the disease.The kinetics of various leucocyte subpopulations in the peripheral blood of animals revealed that leucopenia in response to T. annulata infection was caused by decline in the number of circulating neutrophils and lymphocytes. Lymphocytopaenia was caused mainly by reduction in the levels of circulating CD4+ cells and B cells, and to a lesser degree of CD8+ cells and 7<5 T cells. Cells of the monocyte/macrophage lineage were least affected. The severity of leucopenia was related to the ensuing parasitological reactions after immunisation with cell line or challenge with sporozoites. The recovery and development of immunity in animals after cell line immunisation and sporozoite challenge was associated with an increase in CD25+ cells in the peripheral blood. A transient increase in the levels of CD8+ cells initially followed by a sustained increase in monocytes was associated with recovery indicating that immunity to T. annulata is mainly mediated by mechanisms involving cells of these two types. These cellular changes were not observed in animals undergoing cell line immunisation where the pre-existing allogeneic responses blocked development of parasite specific immune responses.trated on parasite dissemination and T cell activation in the draining efferent lymphatics following sporozoite infection or allogeneic cell line immunisation. During lethal sporozoite infection in the naive animals, the parasite induced strong lymphoproliferation. There was a massive increase in the output of blasting cells in efferent lymph from day 6 with a 5-10 fold increase in the flow rate and cell output. Efferent lymph contained very high levels of interferon-7 between day 5 to 9 after acute sporozoite infection. Parasite infected cells were seen from day 6 onwards. An initial increase in the percentage of CD2+ cells corresponding with an increase in CD4+ cells was observed from day 5. Many CD4+ cells expressed CD25 transiently and became MHC class II+. This was followed by increased output of blasting CD8+ cells with high MHC class II expression. Blasting CD4+ and CD8+ cells gradually lost CD2 expression with the progression of infection. These cells were MHC class II+ but had lost CD25 expression suggesting an inappropriate activation of T cells in response to the parasite. Efferent lymph cells were less responsive to Con. A or exogenous IL-2 stimulation in vitro during the later stages of infection. The cells did not proliferate in vitro in response to autologous parasite infected cells and did not kill autologous parasite infected cells suggesting lymphocyte unresponsiveness.Animals immunised with lxlO6 T. annulata infected cells exhibited a 2-4 fold increase in the flow rate and cell output in efferent lymph. Parasite infected cells of the recipient origin were isolated from day 11 onwards, but cells of donor origin were never isolated. Two peaks of blasting cells were observed. The first corresponded to an allogeneic response against MHC antigens of the immunising cell line and the second was associated with the parasite specific response. However, a few CD4+ and CD8+ cells not expressing CD2 were also observed during the second parasite specific phase of the response. At this stage, CD25 and MHC class II expression on T cells increased. The inhibition of parasite development after cell line immunisation as a result of the pre-existing allogeneic response was not associated with any of these changes.The results indicate that infection with T. annulata sporozoites leading to acute theileriosis induces an inappropriate proliferation and nonspecific activation of T cells in the efferent lymph. T cells initially express normal activation markers but lose CD2, a very important adhesion molecule with the progression of the disease. Both specific and some non-specific T cell activation is initiated after immunisation with T. annulata infected allogeneic cell lines which induce immunity to sporozoite challenge.