Suppression of in vitro IFN-gamma production by spleen cells of Plasmodium chabaudi-infected C57BL/10 mice exposed to dexamethasone at a low dose

Does vitamin D reduce the mortality rate of Plasmodium infection?: a systematic review and meta-analysis

BACKGROUND

Vitamin D supplementation is recommended as an effective adjunct to counteract malaria pathogenesis, but the evidence on this point is limited and controversial. This systematic review and meta-analysis aimed to investigate the effect of vitamin D administration on the survival rate of Plasmodium-infected animals in experimentally-induced malaria on days 6 and 10 post-infection.

METHODS

Five electronic databases were searched up to 20 December 2021. The pooled risks ratio (RR) and associated 95% confidence interval were estimated using the Restricted-maximum likelihood (REML) random-effects model. Heterogeneity was assessed by Cochran's Q test and I² value. Sub-group analyses were used to identify the sources of heterogeneity for several variables, such as type of vitamin D, type of intervention, and dose of vitamin D.

RESULTS

Out of 248 articles found in the electronic database, six were eligible for inclusion in the meta-analysis. The current study found that the pooled random effect of risks ratio favored a statistically significant effect of vitamin D administration on survival rate in infected mice on day 6 post Plasmodium infection (RR = 1.08, 95%CI 1.03, 1.15, p < 0.99; I² = 0%). It also found that vitamin D administration significantly affected the survival rate on day 10 post-infection (RR = 1.94, 95%CI 1.39, 2.71, p < 0.001; I² = 69.02%). Subgroup analyses demonstrated a significant pooled RRs of the positive effect of vitamin D administration for cholecalciferol (RR = 3.11, 95%CI 2.41, 4.03, p < 0.001; I² = 0%), doses higher than 50 µg/kg (RR = 3.37, 95%CI 2.55, 4.27, p < 0.001; I² = 0%), and oral administration (RR = 3.01, 95%CI 2.37, 3.82, p < 0.001; I² = 0%).

CONCLUSION

This systematic review and meta-analysis showed that vitamin D administration positively affects the survival rate in Plasmodium-infected mice. Since, the mouse model may not accurately reproduce the clinical and pathological features of human malaria, future research should investigate the impact of vitamin D in human malaria.

Dual antiplasmodial activity of vitamin D3 and its analog, 22-oxacalcitriol, by direct and indirect mechanisms

Recent evidence suggests that 1α,25-dihydroxyvitamin D3 (calcitriol, VD3), the active form of vitamin D (VD), can inhibit the proliferation of microorganisms. In the present study, we conducted in vitro experiments and utilized in vivo murine models to investigate the antimalarial activity of VD3 and its analog, 22-oxacalcitriol (22-OCT), which was designed to cause less hypercalcemia than VD3. VD3 and 22-OCT treatments effectively resolved a Plasmodium chabaudi (Pc) infection in wild-type mice. Reduced parasitemia was observed during the acute phase of infection in the presence of VD3 and 22-OCT, followed by a delayed peak during the chronic stage of infection. Some anti-Pc activity was observed in VD receptor knockout (KO) mice. VD3 and 22-OCT also completely inhibited the proliferation of P. falciparum (Pf) in human red blood cells in vitro. Plasma levels of interferon (IFN)-γ in VD3-treated B10 and B6 mice were lower than those in vehicle-treated animals, and VD3 resolved a Pc infection in IFN-γ-KO mice, which greatly improved survival. These data suggest that the protective effects of VD3 are elicited through an IFN-γ-independent mechanism. Effective antiplasmodial doses of VD3 and 22-OCT resulted in a loss of body weight in mice. This loss in body weight occurred concomitantly with the development of hypercalcemia. Zoledronic acid partially attenuated VD3-induced hypercalcemia and abrogated the antiparasitic effects of VD3. This study highlights a potential therapeutic role for VD3 in the treatment of malarial infections and shows that hypercalcemia is excellent indicator of the antiplasmodial activity of VD3.

Divergent roles of IRAK4-mediated innate immune responses in two experimental models of severe malaria

Severe malaria represents a clinical spectrum of disease. We propose that innate immune inflammatory responses to malaria play key roles in the pathogenesis and clinical outcomes of distinct severe malaria syndromes. To investigate this hypothesis, mice deficient in IRAK4, central to Toll-like receptor (TLR)-mediated signaling, were studied in two experimental models of malaria: Plasmodium berghei (PbA) and Plasmodium chabaudi (PccAS). Irak4(-/-)mice had decreased pro-inflammatory cytokine production during infection in both models. However, animals were relatively protected from PbA-associated symptoms compared with wild-type mice, whereas Irak4(-/-) animals were more susceptible to PccAS-associated disease. These results show that IRAK4-mediated innate immune inflammatory responses play critical roles in divergent clinical outcomes in murine malaria models. As such, integrated approaches, using more than one model, are required to fully understand the parasite/host interactions that characterize severe malaria, and more importantly, to fully assess the effect of adjunctive therapies targeting innate immune responses to malaria.

Quantifying the relationship between multiple immunological parameters and host resistance: probing the limits of reductionism

Although reductionist experimental designs are excellent for identifying cells, molecules, or functions involved in resistance to particular microbes or cancer cells, they do not provide an integrated, quantitative view of immune function. In the present study, mice were treated with either dexamethasone (DEX) or cyclosporin A (CyA), and immune function and host resistance were evaluated. Multivariate statistical methods were used to describe the relative importance of a broad range of immunological parameters for host resistance in mice treated with various dosages of DEX. Multiple regression and logistic regression analysis indicated that changes in 24 immunological parameters explained a substantial portion of the changes in resistance to B16F10 tumor cells or streptococcus group B. However, at least 40% of the change in host resistance remained unexplained. DEX at all dosages substantially suppressed numerous relevant immunological parameters, but significantly decreased resistance to Listeria monocytogenes only at the highest dosage. In contrast, CyA substantially decreased resistance to L. monocytogenes at dosages that caused relatively minor suppression of just a few immunological parameters (unfortunately, CyA data and host resistance data for L. monocytogenes were not suitable for multivariate analysis). These results illustrate that mathematical models can be used to explain changes in host resistance on the basis of changes in immune parameters, and that moderate changes in relevant immunological parameters may not produce the types of changes in host resistance expected on the basis of results from reductionist experimental designs.

UV-B irradiation increases susceptibility of mice to malarial infection

We here examined whether exposure of mice to UV-B affected their susceptibility to the murine malaria parasite Plasmodium chabaudi. When BALB/c mice with depilated skin were irradiated with UV-B and subsequently infected with the parasite, 80 to 100% of the UV-B-irradiated mice died within 12 days of infection with a sublethal dose. In addition, UV-B irradiation of C57BL/10 (B-10) mice, which are otherwise naturally resistant to the parasites, rendered them susceptible, and 100% of irradiated B-10 mice died within 11 days postinfection. The level of plasma gamma interferon (IFN-gamma) in unirradiated B-10 mice at 5 days after infection increased to 566 pg/ml, whereas the UV-B exposure of mice impaired the production of IFN-gamma, which showed a maximum level of 65 pg/ml in response to the parasite infection. The maximum level of plasma interleukin-10 in UV-B-irradiated mice in response to the parasite infection was approximately 1,100 pg/ml, which was approximately fourfold higher than the maximum level in unirradiated control mice. When UV-B-irradiated B-10 mice were administered murine recombinant IFN-gamma after infection, the mice regained parasite resistance. These results demonstrated that the UV-B exposure of mice enhances the susceptibility to the malaria parasites and suggested that the enhanced susceptibility following UV-B exposure was mediated by impairment of IFN-gamma production in response to the parasite infection.

Transforming growth factor beta-induced failure of resistance to infection with blood-stage Plasmodium chabaudi in mice

The role of transforming growth factor beta (TGF-beta) in infection with Plasmodium chabaudi was investigated with resistant and susceptible mouse models. C57BL/10 mice produced gamma interferon (IFN-gamma) and nitric oxide (NO) shortly after infection and cleared the parasite spontaneously. In contrast, BALB/c mice showed a transient enhancement of TGF-beta production, followed by a relative lack of IFN-gamma and NO production, and succumbed to the infection. However, there was no correlation between levels of serum TGF-beta and splenic TGF-beta mRNA in both mouse strains before and after infection. Administration of recombinant TGF-beta (rTGF-beta) rendered resistant mice susceptible because of suppression of subsequent production of IFN-gamma and NO. Administration of anti-TGF-beta antibody to the infected BALB/c mice resulted in remarkable increases in serum IFN-gamma and NO, and the mice resisted the infection. Splenic CD4(+) T and CD11b+ cells of C57BL/10 mice were significantly activated after infection, but this was completely abrogated by administration of rTGF-beta. These results suggested that, in the P. chabaudi-susceptible but not resistant mice, production of TGF-beta was promoted, and subsequent failure of IFN-gamma- and NO-dependent resistance to the parasite was induced. This study is the first to indicate that TGF-beta production was the key event in failure of resistance to mouse malaria.