Specific and nonspecific mediation of protective immunity to Toxoplasma gondii

Immunization of BALB/c Mice with Killed Tachyzoites of Toxoplasma gondii against Acute Toxoplasmosis

Background

Toxoplasma gondii with widespread distribution infects over one third of human populations in the world and can cause serious life-threatening diseases especially for the immunodeficient patients in acute toxoplasmosis. As the clinical pharmaceutical drugs with severe side effects for treatment and non-ideal extant vaccines for prevention, more work starves to be done for keeping advantages in the athletics.

Methods

Aluminum adjuvant and hybrid formaldehyde-killed tachyzoites of T. gondii RH and GT1 isolates were prepared to intramuscularly immunize BALB/c mice for five times at 0, 3, 7, 14 and 21 days post first injection. The triggered humoral and cellular immune responses at two weeks post the last immunization and the survival times of infected mice were examined for the hybrid immunization scheme judgement.

Results

The anti-RH and anti-GT1 specific antibodies were both increased at one week prior to challenge (P < 0.05), and the survival times of immunized mice (7.33 ± 0.71 d for RH, 7.22 ± 0.97 d for GT1) against acute toxoplasmosis were significantly prolonged by the immunizations performed in the study compared to blank control (6.67 ± 0.50 d for RH, 6.33 ± 0.71 d for GT1; P < 0.05), with the higher IFN-γ, IL-2 and IL-12p70 in sera, the elevated CD3e+CD4+ T and CD3e+CD8a+ T cells, and the enhanced lymphocyte proliferation in spleen (P < 0.05).

Conclusion

The hybrid killed tachyzoites with aluminum adjuvant induced humoral and cellular immune responses of mice, and offered mildly protective efficacy against acute toxoplasmosis.

Recombinant attenuated Toxoplasma gondii expressing the Plasmodium yoelii circumsporozoite protein provides highly effective priming for CD8+ T cell-dependent protective immunity against malaria

The protozoan parasite Toxoplasma gondii elicits strong cell-mediated immunity against itself as well as nonspecific resistance against other pathogens and tumors. For this reason, we asked whether recombinant Toxoplasma could be utilized as an effective vaccine vehicle for inducing immunity against heterologous microbial infections. The circumsporozoite protein (PyCSP) of Plasmodium yoelii was engineered into a T. gondii temperature-sensitive strain (ts-4), a mutant that induces complete protection against virulent Toxoplasma challenge. When administered to mice in a single dose, a recombinant ts-4 (CSC3) that both secretes and expresses surface PyCSP induced strong anti-CSP Ab responses, with an isotype distribution pattern similar to that stimulated by the T. gondii carrier. When challenged with P. yoelii sporozoites during the first month after CSC3 vaccination, these animals displayed substantial levels of nonspecific resistance attributable entirely to the T. gondii carrier. Nevertheless, after the nonspecific protection had waned, high levels (up to 79%) of specific immunity against sporozoite challenge were achieved by boosting the animals with recombinant vaccinia virus expressing PyCSP. These CSC3-primed PyCSP-vaccinia-boosted mice displayed high frequencies of splenic PyCSP-specific IFN-gamma-producing cells, as well as CD8+ T cell-dependent cytolytic activity. In vivo depletion of CD8+ lymphocytes at the time of challenge completely ablated protective immunity in the T. gondii-primed/vaccinia-boosted animals, while neutralization of IFN-gamma or IL-12 caused a partial but significant reduction in resistance. Together these findings establish the efficacy of recombinant attenuated Toxoplasma as a vaccine vehicle for priming CD8+-dependent cell-mediated immunity.

Protective role for interleukin-5 during chronic Toxoplasma gondii infection

To investigate the role of interleukin-5 (IL-5) during Toxoplasma gondii infection, IL-5 knockout (KO) mice and C57BL/6 control mice were infected intraperitoneally with ME49 cysts and the course of infection was monitored. The mortality rate during chronic infection was significantly greater in IL-5-deficient animals, and consistent with this finding, the KO mice harbored a greater number of brain cysts and tachyzoites than did their wild-type counterparts. Although the IL-5 KO animals did not succumb until late during infection, increased susceptibility, as measured by accelerated weight loss, was detectable during the acute stages of infection. The amounts of total immunoglobulin (Ig), IgM, and IgG2b were comparable in both strains, while the amount of IgG1 was much smaller in IL-5 KO mice. Spleen cell production of IL-12 in response to T. gondii antigen was approximately threefold lower in the KO strain, and this decrease correlated with a selective loss of B lymphocytes during culture. A link between the presence of B cells and augmented IL-12 production was established by the finding that after removal of B cells with monoclonal antibody and complement, wild-type- and KO-derived cells produced equivalent levels of IL-12 in response to T. gondii antigen. These results demonstrate a protective role of IL-5 against T. gondii infection and suggest that IL-5 may play a role in the production of IL-12.

Regulation and function of T-cell-mediated immunity during Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii is a widespread opportunistic parasite of humans and animals. Normally, T. gondii establishes itself within brain and skeletal muscle tissues, persisting for the life of the host. Initiating and sustaining strong T-cell-mediated immunity is crucial in preventing the emergence of T. gondii as a serious pathogen. The parasite induces high levels of gamma interferon (IFN-gamma) during initial infection as a result of early T-cell as well as natural killer (NK) cell activation. Induction of interleukin-12 by macrophages is a major mechanism driving early IFN-gamma synthesis. The latter cytokine, in addition to promoting the differentiation of Th1 effectors, is important in macrophage activation and acquisition of microbicidal functions, such as nitric oxide release. During chronic infection, parasite-specific T lymphocytes release high levels of IFN-gamma, which is required to prevent cyst reactivation. T-cell-mediated cytolytic activity against infected cells, while easily demonstrable, plays a secondary role to inflammatory cytokine production. While part of the clinical manifestations of toxoplasmosis results from direct tissue destruction by the parasite, inflammatory cytokine-mediated immunopathologic changes may also contribute to disease progression.

Does human toxoplasmosis involve an imbalance in T1/T2 cytokines?

The T1 (interferon-gamma, interleukin-12, interleukin-2) and T2 (interleukin-4, interleukin-10, interleukin-6) cytokine groups constitute two polar responses of the immune system. The T1 group is a predominantly cellular response, while the T2 group response is mainly humoral. The hypothesis forwarded here links these subgroups of induced cytokines to the various clinical forms of human toxoplasmosis. Ocular toxoplasmosis in immunocompetent patients could be attributed to a T1 hyper-response, whereas congenital toxoplasmosis, toxoplasmic encephalitis (in immunodeficient patients) and active chronic toxoplasmosis (with persistent lymphadenophathy) would be characterized by a predominantly T2 response. Confirmation that this kind of immunological imbalance effectively underlies the various clinical forms of toxoplasmosis would open the way for a new range of treatments based on immunomodulation.

Correlation between increased susceptibility to primary Toxoplasma gondii infection and depressed production of gamma interferon in pregnant mice

To explore a possible mechanism of pregnancy-associated suppression of T cell-mediated immunity to Toxoplasma gondii, acquired resistance and gamma interferone (IFN-gamma) production in pregnant mice were compared with those in virgin mice after infection with the S-273 strain of this protozoan parasite. The 50% lethal dose of this strain was less than 200 tachyzoites for pregnant mice and 2,800 organisms for virgin controls. Toxoplasma-induced production of both IFN-alpha and IFN-gamma in the bloodstream of pregnant mice was significantly depressed as compared with that in virgin controls. The administration of recombinant murine IFN-gamma (rMuIFN-gamma) resulted in a significant decrease of mortality and parasitic growth in the organs of pregnant mice infected with a lethal dose of S-273 strain tachyzoites. Thus, the impairment of T cell-mediated immune responses was evident in pregnant mice from the impaired IFN-gamma-generating capacity and poor survival rate after primary infection with Toxoplasma. When mice with chronic Toxoplasma infection were injected with specific antigen, the resultant production of IFN-gamma was also significantly suppressed during pregnancy. However, there was no direct correlation between the serum levels of IFN-gamma and susceptibility to reinfection, since the mortality rate of chronically infected pregnant mice after the challenge with the high virulent RH strain was not significantly higher than that of virgin controls.

Human T-cell clones against Toxoplasma gondii: production of interferon-gamma, interleukin-2, and strain cross-reactivity

The soluble fraction from a sonicate of Toxoplasma gondii tachyzoites (termed F3) was shown to induce dose-dependent blastic transformation of peripheral-blood mononuclear cells (PBMC) from seropositive individuals only and was used to isolate a panel of T-cell clones from the PBMC of an immune donor. Proliferation assays using F3 showed that 15 (14 CD4+ and 1 CD8+) of the 18 isolated clones were specific for T. gondii. In response to antigen stimulation, 5 of the 15 clones produced detectable levels of interleukin-2 (IL-2, 0.2-15 u/ml) and 9 clones produced significant levels of interferon-gamma (IFN-gamma, 17.5-1400 IU/ml). Seven of the 7 T-cell clones tested reacted with two different Toxoplasma strains (RH and Wiktor). When used as antigen-presenting cells, an autologous B-lymphoblastoid cell line could efficiently present the antigen to only three of the six T-cell clones tested. This study identifies and characterizes cellular probes that could be useful for future vaccine design.

Protozoan infections

Protozoan infections, against which immunity is predominantly T cell mediated, are likely to be more severe in patients with the acquired immune deficiency syndrome (AIDS) than in immunocompetent hosts. Leishmaniasis, toxoplasmosis and cryptosporidiosis are examples, the last two being particularly common in AIDS patients. Cerebral toxoplasmosis almost always results from recrudescence of latent infections acquired earlier in life. Depletion of T-helper (CD4+) lymphocytes enables bradyzoites to survive if released from cysts in the brain of patients. In the absence of immune pressure bradyzoites revert to tachyzoites and multiply to cause a rapidly developing, necrotizing encephalitis which needs immediate treatment. AIDS patients, especially those who are negative for antibodies to Toxoplasma, should avoid cats, the source of oocysts, and undercooked meat which may contain tissue cysts, as primary infections may become systemic. Cryptosporidium infections are more likely to be primary infections. Sources of infection are other people, farm animals and pets and there is a significant risk from contaminated domestic water supplies. As infections cause a life-threatening secretory diarrhoea in AIDS patients, for which there is not satisfactory treatment at present, such patients should take steps to minimize the risk of infection.

Eimeria tenella and E. acervulina: lymphokines secreted by an avian T cell lymphoma or by sporozoite-stimulated immune T lymphocytes protect chickens against avian coccidiosis

The role of avian lymphokines as nonspecific immunomodulators of host immunity against the intracellular parasite Eimeria was investigated. Prophylactic treatment of normal chickens with crude cell-free supernatants obtained from JMV-1 culture, concanavalin A (Con A)-stimulated normal spleen cells, or sporozoite-stimulated immune T cells prior to inoculation with E. tenella or E. acervulina conferred significant protection. These crude cell-free culture supernatants also inhibited intracellular development of eimerian parasites in vitro. Avian macrophages pretreated with these supernatant preparations showed inhibitory activity against Eimeria. This inhibitory activity could not be ascribed to anti-Eimeria antibody, complement, or cell-free Marek's disease virus and was therefore considered to be due to immunomodulating lymphokines present in the culture supernatants. These results suggest that JMV-1-transformed T lymphoblastoid cells, immune T lymphocytes, and Con A-stimulated normal spleen cells secrete lymphokines that can enhance host immunity in a nonspecific manner and implicate cell-mediated immunity as a major mechanism of the protective host immune response against eimerian infections.

Efforts towards a vaccine against Toxoplasma gondii: a review

In a review, past as well as present investigations carried out towards a vaccine against toxoplasmosis are outlined. A historical retrospect of the various immunization experiments is given, recent research projects intending the characterization of antigens that are relevant to host protective immunity are described, and a prospect to future problems and developments expected in the field is drafted.

Pathophysiology of toxoplasmosis

Toxoplasma infection in most adult animals and humans is asymptomatic because of effective protective immunity; this involves antibody acting extracellularly, and T-cell factors acting intracellularly. Whenever immunity is not acquired in a timely fashion, tachyzoites continue to multiply, destroying an excessive number of cells, producing lesions in several organs, with pneumonia and encephalitis the prominent causes of illness and death. However, immunity is insufficient to destroy the slowly multiplying bradyzoites persisting in tissue cysts in many organs - a parasite adaptation to await ingestion of one host by another. Toxoplasma cysts produce lesions when they disintegrate, because of the delayed type of hypersensitivity accompanying infections. In the presence of immunity, the released bradyzoites are destroyed, but when protective immunity fails, the bradyzoites can develop again into actively multiplying tachyzoites parasitizing and destroying cells in expanding foci, usually in the brain. In this review J.K. Frenkel discusses the complex interplay of immunological and parasite factors participating in the various lesions associated with acute and chronic Toxoplasma infections.