Immunoprotection in mice susceptible to waning memory against the pre-erythrocytic stages of malaria after validated immunisation with irradiated sporozoites of Plasmodium berghei

Reactivity of autoantibodies against not only erythrocytes but also hepatocytes in sera of mice with malaria

In order to further examine the reactivity of autoantibodies, mice were infected with a non-lethal strain of Plasmodium yoelii. Parasitemia appeared between days 10 and 21. During this period, hyperglycemia and hypothermia were serially obeserved and this phenomenon resembled stress-associated responses. In parallel with parasitemia, autoantibodies appeared against nucleus and double-stranded DNA in the sera. To examine further the reactivity of autoantibodies against tissues, immunohistochemical staining using sera from mice with or without malaria was conducted. Autoantibodies contained reactivity to erythrocytes in the spleen, bone marrow and peripheral blood, especially against tissues obtained from mice with malaria. In the liver and intestine, autoantibodies reacted with hepatocytes and intestinal epithelial cells, respectively. These results suggested that the reactivity of autoantibodies against erythrocytes and hepatocytes might be associated with the modulation of the disease course in malaria.

Differential effector pathways regulate memory CD8 T cell immunity against Plasmodium berghei versus P. yoelii sporozoites

Malaria results in >1,000,000 deaths per year worldwide. Although no licensed vaccine exists, much effort is currently focused on subunit vaccines that elicit CD8 T cell responses directed against Plasmodium parasite liver stage Ags. Multiple immune-effector molecules play a role in antimicrobial immunity mediated by memory CD8 T cells, including IFN-gamma, perforin, TRAIL, Fas ligand, and TNF-alpha. However, it is not known which pathways are required for memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. In this study, we used a novel immunization strategy to generate memory CD8 T cells in the BALB/c mouse model of P. berghei or P. yoelii sporozoite infection to examine the role of immune-effector molecules in resistance to the liver stage infection. Our studies reveal that endogenous memory CD8 T cell-mediated protection against both parasite species is, in part, dependent on IFN-gamma, whereas perforin was only critical in protection against P. yoelii. We further show that neutralization of TNF-alpha in immunized mice markedly reduces memory CD8 T cell-mediated protection against both parasite species. Thus, our studies identify IFN-gamma and TNF-alpha as important components of the noncytolytic pathways that underlie memory CD8 T cell-mediated immunity against liver stage Plasmodium infection. Our studies also show that the effector pathways that memory CD8 T cells use to eliminate liver stage infection are, in part, Plasmodium species specific.

Memory CD8 T cell responses exceeding a large but definable threshold provide long-term immunity to malaria

Infection of mice with sporozoites of Plasmodium berghei or Plasmodium yoelii has been used extensively to evaluate liver-stage protection by candidate preerythrocytic malaria vaccines. Unfortunately, repeated success of such vaccines in mice has not translated readily to effective malaria vaccines in humans. Thus, mice may be used better as models to dissect basic parameters required for immunity to Plasmodium-infection than as preclinical vaccine models. In turn, this basic information may aid in the rational design of malaria vaccines. Here, we describe a model of circumsporozoite-specific memory CD8 T cell generation that protects mice against multiple P. berghei sporozoite challenges for at least 19 months. Using this model we defined a threshold frequency of memory CD8 T cells in the blood that predicts long-term sterilizing immunity against liver-stage infection. Importantly, the number of Plasmodium-specific memory CD8 T cells required for immunity greatly exceeds the number required for resistance to other pathogens. In addition, this model allowed us to identify readily individual immunized mice that exceed or fall below the protective threshold before infection, information that should greatly facilitate studies to dissect basic mechanisms of protective CD8 T cell memory against liver-stage Plasmodium infection. Furthermore, the extremely large threshold in memory CD8 T cell frequencies required for long-term protection in mice may have important implications for development of effective malaria vaccines.

Essential role of extrathymic T cells in protection against malaria

Athymic nude mice carry neither conventional T cells nor NKT cells of thymic origin. However, NK1.1(-)TCR(int) cells are present in the liver and other immune organs of athymic mice, because these lymphocyte subsets are truly of extrathymic origin. In this study, we examined whether extrathymic T cells had the capability to protect mice from malarial infection. Although B6-nu/nu mice were more sensitive to malaria than control B6 mice, these athymic mice were able to survive malaria when a reduced number of parasitized erythrocytes (5 x 10(3) per mouse) were injected. At the fulminant stage, lymphocytosis occurred in the liver and the major expanding lymphocytes were NK1.1(-)TCR(int) cells (IL-2Rbeta(+)TCRalphabeta(+)). Unconventional CD8(+) NKT cells (V(alpha)14(-)) also appeared. Similar to the case of B6 mice, autoantibodies (IgM type) against denatured DNA appeared during malarial infection. Immune lymphocytes isolated from the liver of athymic mice which had recovered from malaria were capable of protecting irradiated euthymic and athymic mice from malaria when cell transfer experiments were conducted. In conjunction with the previous results in euthymic mice, the present results in athymic mice suggest that the major lymphocyte subsets associated with protection against malaria might be extrathymic T cells.

Extrathymic T cells in malaria protection, including evidence for the onset of erythropoiesis in the liver during infection

This review proposes the possibility that malarial protection might not be achieved through the process of acquired immunity in which the constituents are conventional T and B (B-2) cells. On the other hand, malarial protection might be achieved by the process of innate immunity in which the constituents are extrathymic T cells and autoantibody-producing B-1 cells. Accordingly, mice infected with malaria exhibited severe thymic atrophy, and the expansion of IL-2Rbeta+ CD3int cells and its subset of NK1.1-CD3int cells were simultaneously induced. In parallel with the expansion of extrathymic T cells in the liver, extramedullary erythropoiesis was found to begin in the liver of these mice. Interestingly, malarial protozoa were primarily seen in only these nucleated erythrocytes in the liver at the early stage of infection. These results suggest that malaria immunology falls into a new field of immunology, namely, innate immunity. The similarity of the immune states among malaria, aging, and autoimmune diseases also suggest that the immunosuppression of a conventional, acquired immune system is more likely the common mechanism underlying these diseases or physiological responses.

Complete development of mosquito phases of the malaria parasite in vitro

Methods for reproducible in vitro development of the mosquito stages of malaria parasites to produce infective sporozoites have been elusive for over 40 years. We have cultured gametocytes of Plasmodium berghei through to infectious sporozoites with efficiencies similar to those recorded in vivo and without the need for salivary gland invasion. Oocysts developed extracellularly in a system whose essential elements include co-cultured Drosophila S2 cells, basement membrane matrix, and insect tissue culture medium. Sporozoite production required the presence of para-aminobenzoic acid. The entire life cycle of P. berghei, a useful model malaria parasite, can now be achieved in vitro.

Resistance to malarial infection is achieved by the cooperation of NK1.1(+) and NK1.1(-) subsets of intermediate TCR cells which are constituents of innate immunity

We previously reported that the major expanding lymphocytes were intermediate TCR (TCR(int)) cells (mainly NK1.1(-)) during malarial infection in mice. Cell transfer experiments of TCR(int) cells indicated that these T cells mediated resistance to malaria. However, TCR(int) cells always contain NK1.1(+)TCR(int) cells (i.e., NKT cells) and controversial results (NKT cells were effective or not for resistance to malaria) have been reported by different investigators. In this study, we used CD1d((-/-)) mice, which almost completely lack NKT cells in the liver and other immune organs. Parasitemia was prolonged in the blood of CD1d((-/-)) mice and the expansion of lymphocytes in the liver of these mice was more prominent after an injection of Plasmodium yoelii-infected erythrocytes. However, these mice finally recovered from malaria. In contrast to B6 mice, CD4(-)8(-) NKT cells as well as NK1.1(-)CD3(int) cells expanded in CD1d((-/-)) mice after malarial infection, instead of CD4(+) (and CD8(+)) NKT cells. These newly generated CD4(-)8(-)NKT cells in CD1d((-/-)) mice did not use an invariant chain of Valpha14Jalpha281 for TCRalpha. Other evidence was that severe thymic atrophy and autoantibody production were accompanied by malarial infection, irrespective of the mice used. These results suggest that both NK1.1(-) and NK1.1(+) subsets of TCR(int) cells (i.e., constituents of innate immunity) are associated with resistance to malaria and that an autoimmune-like state is induced during malarial infection.

Association of intermediate T cell receptor cells, mainly their NK1.1(-) subset, with protection from malaria

Mice were infected with Plasmodium (P.) yoelii blood-stage parasites. Both the liver and spleen were the sites of inflammation during malarial infection at the beginning of day 7. The major expanding cells were found to be NK1.1(-) intermediate alphabetaTCR (alphabetaTCR(int)) in the liver and spleen, although the population of NK1.1(+) alphabetaTCR(int) cells remained constant or slightly increased. These TCR(int) cells are of extrathymic origin or are generated by an alternative intrathymic pathway and are distinguished from conventional T cells of thymic origin. During malarial infection, the population of conventional T cells did not increase at all. TCR(int) cells purified from the liver of mice which had recovered from P. yoelii infection protected mice from malaria when they were transferred into 6.5-Gy-irradiated mice. Interestingly, the immunity against malaria seemed to disappear as a function of time after recovery, namely, mice which had recovered from malaria 1 year previously again became susceptible to malarial infection. The present results suggest that TCR(int) cells are intimately associated with protection against malarial infection and, therefore, that mice which had recovered from malaria 1 year previously lost such immunity.

Susceptibility of different strains of mice to hepatic infection with Plasmodium berghei

Despite the low susceptibility of BALB/c mice to hepatic infection by Plasmodium berghei, this animal model is routinely used to investigate the basic biology of the malaria parasite and to test vaccines and the immune response against exoerythrocytic (EE) stages derived from sporozoites. A murine model in which a large number of EE parasites are established would be useful for furthering such investigations. Therefore, we assayed six mouse strains for susceptibility to erythrocytic and hepatic infections. The administration of 50 sporozoites by intravenous inoculation was sufficient to establish erythrocytic infections in five of five C57BL/6 mice compared with 10,000 sporozoites required to infect 100% of BALB/c mice. To assay for hepatic infections, mice received an intravenous inoculum of 10(6) sporozoites, and liver sections for light microscopy and histology were obtained at 29 and 44 h postinoculation. EE parasites were visualized by immunofluorescence, using an antibody to a P. falciparum heat shock protein. The mean number of EE parasites per 100 cm2 for C57BL/6 and A/J strains was significantly higher than that for BALB/c (2,190 +/- 260, 88 +/- 38, and 6 +/- 2, respectively). The proportion of inoculated sporozoites transforming into liver schizonts was 8.2% in C57BL/6 and < 1% in C3H/HeJ, DBA/1, and Swiss CD-1/ICR mice. Nonspecific inflammatory infiltrates around EE parasites were less prevalent in liver sections from C57BL/6 mice than in those from BALB/c mice, which contributed to the decrease in developing EE stages in BALB/c mice. These data indicate that the C57BL/6-P. berghei system is preferable for investigating the biology and immunology of liver stage parasites.