Search

A single dose of a formalin-fixed malaria vaccine which normally protects mice against challenge with the live parasite, is ineffective in mice born to immune mothers. This inhibition of protection, which is due to maternally-derived IgG, can be overcome if a second dose of vaccine is given 10 or more days after the first. We show that this is related to the production of specific IgM antibody in response to the first dose of vaccine, which competitively blocks the inhibitory effect of the IgG. The implications of this finding in relation to immune regulation and immunization regimes are discussed.

Strain differences in ease of induction of autoantibody production were observed when mice were injected with rat RBC. Responsiveness was not linked to the H-2 locus. NZB and (NZB X BALB/c)F1 mice were hyper-responsive both in terms of the induction of autoantibody and in the production of agglutinating antibody to rat RBC. C57BL/c and (C57BL X BALB/c)F1 were poor responders. Injection of the rat RBC in FCA converted a poor responder into a good responder. Adult thymectomy and ALS treatment did not significantly enhance autoantibody production.

Heat-stable soluble products of rodent malarial parasites induce activated peritoneal macrophages to secrete tumour necrosis factor (TNF) in vitro. Since heat-stable parasite antigens are known to be present in the circulation of patients with malaria and it has been suggested that much of the pathology of malaria is due to TNF, we investigated the ability of such antigens to induce the production of TNF in vivo and to be toxic to mice. Injection of antigens obtained from Plasmodium yoelii or from P. berghei into mice which had previously received the macrophage-activating agent Propionibacterium acnes induced the release of TNF into the serum in amounts equivalent to the maximum release induced by bacterial lipopolysaccharide (LPS). Specific antiserum blocked the ability to the boiled soluble antigens, but not of LPS, to induce release of TNF. Similarly, vaccination specifically inhibited the release of TNF into the serum in response to subsequent stimulation with the antigens, but not with LPS. Mice made hypersensitive to the lethal action of TNF by pretreatment with D-galactosamine were killed in a dose-related fashion by administration of antigen preparations; addition of specific antiserum or prior vaccination with the antigens protected such mice, but not those given LPS, from death. We conclude that, in malaria, soluble antigens derived from the parasites may act like a toxin by stimulating the production of TNF, an important mediator of endotoxic shock, and that immunization with such antigens may diminish TNF secretion and consequently many of the clinical manifestations of the disease.

Serum from normal mice contains an inhibitor of interleukin-2 (IL-2) which probably interacts directly with IL-2. Athymic mice and normal mice kept under specific pathogen-free conditions do not show this activity, whereas mice infected with malaria parasites have increased serum levels of inhibitor. This IL-2 inhibitor may play an important part in regulating T-cell function.

Female BALB/c mice were vaccinated against blood stage P. yoelii (17XL strain), infected 2 weeks later and after recovery mated to normal C57B1/6 males. Control matings were with normal BALB/c females. The (C57B1/6 x BALB/c)F1 progeny were vaccinated at 4, 6, 8 or 10 weeks of age and infected 2 weeks later with lethal P. yoelii. All control mice were fully protected, but in the offspring of immune mothers mortality was 100, 87, 50, and 0% respectively. Mice in which the protective effect of vaccination had been abolished showed greatly reduced specific IgG and delayed hypersensitivity (DH) responses to challenge with parasite antigen. Results indicate that this failure of vaccination is due to the transmission of maternal IgG to the offspring which acts to suppress both priming by the vaccine and the generation of specific T helper cells involved in IgG production, as measured by the response to TNP-P. yoelii.

A study to investigate the participation of T cells in macrophage-mediated responses during malaria was performed in nude (nu/nu) and littermate (nu/+) mice infected with Plasmodium berghei (PB). We found that in both groups of mice spleen cells suppressed the mitogenic response to LPS. Both nu/+ and nu/nu infected mice also showed liver macrophage activation, reflected by increased plasminogen activator release. These findings suggest that at least some of the macrophage changes during malaria infection are T-independent.

The molecular characteristics of a serum IL-2 inhibitor were determined by fractionating active sera from normal and malaria-infected mice, and assaying inhibitory activity in blocking production and expression of IL-2 activity. Using isoelectric focusing and ion exchange chromatography, the activity resolved in a single peak (pI = 6.2). HPLC gel filtration resolved two peaks of activity (50,000 and 25,000 MW) which are probably related. The molecule is precipitated by 50% saturated ammonium sulphate, and is destroyed by heating above 56 degrees or by acidification below pH 4.

Mice were protected against lethal Plasmodium yoelii malaria by vaccination with a Triton X-100 lysate of whole parasitized erythrocytes. For full effectiveness this vaccine required an adjuvant, and we have found that recombinant gamma-interferon has strong adjuvanticity in this model when given either intraperitoneally or subcutaneously. Specific immune responses that were enhanced included antibody, T cell help, and delayed hypersensitivity.

Normal lymphocytes labelled with 51Cr were injected into mice at various stages of lethal and non-lethal malaria infections. Marked alterations were seen in the uptake into spleen and liver, which correlated with the outcome of the infection. Non-lethal infections and lethal infections in mice protected by vaccination caused increased uptakes, especially in the liver. In lethal infections, particularly Plasmodium berghei, uptakes were below normal values at certain times: this was apparently due to destruction of lymphocytes, probably caused by autoantibody.

The affinity of anti-DNP antibodies produced by mice of various ages has been studied at the cellular level by the plaque inhibition technique. The affinity of PFC produced shortly after a single injection of antigen was found to increase during the first 6 weeks of life. The responses of 1- and 2-week-old animals also showed apparently restricted heterogeneity. The difference in the affinity of anti-DNP--PFC between young and adult mice could not be attributed to different (serum) levels of antigen or to differences in the rate of maturation of affinity during the immune response. Cell transfer experiments suggested that the age-dependent increase was due to a change in the population of antibody-forming (B) cell precursors and not to a progressive improvement of T-cell function. This findings is interpreted as favouring somatic mutation theories of antibody diversity.

A method is described for detecting the simultaneous presence of Fc and C3 receptors on mouse spleen cells. A proportion of both T cells and non-T cells bear both receptors. Both T-cell and non-T-cell Fc receptors were blocked by aggregated mouse IgG2 to the same degree. C3, but not Fc, receptors were blocked by factors present in the serum of irradiated mice or mice undergoing graft-versus-host reaction. Thymocytes activated by injection into irradiated F1 hybrid mice, and thymocytes regenerating after irradiation and bone marrow injection, appeared to have increased Fc receptors. A general role for Fc and C3 receptors in T cell-B cell co-operation is suggested.

Mice were injected with silica or Corynebacterium parvum, which, respectively, inhibit and stimulate macrophages in vivo, in an attempt to study the role of macrophages in lethal Plasmodium yoelii infection and in mice protected by immunization. In the normal infection, macrophages were able to control parasitaemia for up to 1 week, whereas in immunized mice they appeared to inhibit the sterilizing immune response. A model is proposed in which this dual role of activated macrophages may account for the chronic non-sterilizing course of natural malaria infections.

A purified 230,000 mol wt protein antigen from the lethal mouse malaria parasite Plasmodium yoelii YM which had previously been shown to be highly effective as a vaccine, was tested for its ability to stimulate specific helper T cells and T cells responsible for delayed hypersensitivity. Strong stimulation was found in both assays, but larger doses were required for delayed hypersensitivity, correlating well with the requirements for protection. It is suggested that T stimulation may be a requirement for effective protection by purified antigens in malaria.

The effect of pre-immunization with soluble haptenated serum proteins or haptenated carbohydrates on the subsequent responses to the hapten in thymus-dependent or thymus-independent form was investigated. In certain cases both thymus-dependent and -independent responses were suppressed whilst in others a differential suppression of thymus-independent responsiveness was observed. These results are discussed in terms of B-cell subpopulations and mechanisms of tolerance induction.

A comparative study of non-specific immunosuppression by malaria has been carried out in five situations: in both unvaccinated and vaccinated mice infected with the lethal Plasmodium yoelii or the lethal Plasmodium berghei, and in the unvaccinated non-lethal P. yoelii infection. Spleen cells showed a suppressive effect on the normal blastogenic response to mitogens. This suppression was strongest in the mice vaccinated before infection with the lethal P. yoelii and in those infected with non-lethal P. yoelii, suggesting that the suppressive effect did not interfere with recovery. Silica, anti-Thy-1, and indomethacin treatment suggested that this suppression was caused by macrophages. However, the plaque-forming cell response to sheep RBC in vivo was suppressed equally in every case at the peak of the parasitaemia, whereas the suppression of contact sensitivity to oxazolone was strongest in mice with fatal infections. We suggest that different suppressor mechanisms operate in malaria, some being harmful to the host and others possibly beneficial.

(C57BI x Balb/c)F1 mice are normally killed by a strain of the malaria P. yoelii, but they can be fully protected by a killed vaccine. The best results were obtained with saponin-lysed parasitized cells subsequently fixed with 0-06% formalin and injected intravenously with the adjuvant B. pertussis, though good protection was also obtained without the adjuvant. The protection was specific and at least partly mediated by a serum factor. A similar regime gives little or no protection against P. berghei. Possible reasons for this difference are considered and the mechanisms by which vaccination works against P. yoelii are discussed.

Following injection of rat red cells, mice develop anti-red cell autoantibodies and subsequently suppressor T cells specific for these. Likewise, following recovery from non-lethal malaria, they develop suppressor T cells which suppress the anti-lymphocyte autoantibodies induced by lethal malaria parasites. Neither type of suppressor cell affected non-autoantibody components of the response, nor a response to sheep red cells. However, there was variable but significant cross-suppression of the respective autoantibody responses by both types of suppressor cell. Possible reasons for this unexpected cross-reaction are discussed.

Mice vaccinated with a formalin-fixed preparation of either Plasmodium berghei or P. yoelli exhibited delayed type hypersensitivity (DTH) to the homologous antigen. This manifested itself in increased delayed thickening of antigen-challenged pinnae of the vaccinated mice as compared to the non-vaccinated controls. DTH was also evident in the vaccinated mice using the homing of radio-labelled bone marrow cells (BMC) to the delayed lesion as a criterion of reactivity. When P. yoelii vaccinated mice were given a live infection P. yoelii, a marked migration of BMC into the spleen occurred, with a peak at 48 hr, and it is suggested that this was a systemic response of DTH. The splenic T-cells of P. yoelii-vaccinated animals transformed in vitro with a soluble extract of the homologous parasite. The potential function of cell-mediated mechanisms in immunity to malarial infections is discussed.

Irradiated (NZB×BALB/c)F1 mice were injected with syngeneic bone marrow cells, syngeneic or parental thymus cells, and sheep red cells. The antibody plaque-forming cell response depended on the number of sheep cells and the age and strain of the thymus cells. Young or adult BALB/c thymus, and young hybrid thymus, responded best to low numbers of sheep cells; with higher numbers they became tolerant. Adult hybrid thymus, and young or adult NZB thymus, responded better to high numbers of sheep cells. Hybrid mice irradiated and restored with BALB/c bone marrow developed thymus cells with the reactivity of BALB/c thymus. It is argued that NZB mice, and older hybrids, may develop autoimmunity because of an abnormality of tolerance induction manifested in their thymus cells, but of bone marrow origin.

The response of bone marrow (B) cells to sheep red blood cells (SRBC) was measured in the presence of thymus-derived (T) cells that had been stimulated by SRBC or RBC from other species. It was found that other RBC, notably pig and rabbit, were much more effective in stimulating T cells for a cooperative response to SRBC than would have been predicted from the very slight cross-reaction between these RBC and SRBC at the antibody level. It is concluded that recognition of antigens at the T-cell level is different, and in this case less specific, than at the level of the B cells.

During blood-stage infection of mice with a lethal variant of Plasmodium yoelii, cells in both spleen and liver became activated to reach a peak at day 5. In mice protected by vaccination, activation was accelerated after infection. The most striking difference observed was in the 10-fold greater yield of infiltrating cells, including macrophages, obtained from the liver just before the mice recovered. Their capacity to give an oxidative burst and their cytotoxic activity against tumour cells was also more than 10 times normal. This suggests that the recruitment of inflammatory cells to the liver plays an important role in the protection of vaccinated mice against malaria.