疟原虫抗原的免疫识别

为了阐明机体对疟原虫抗原的免疫反应性,作者分析了主要组织相容性复合物(MHC)对疟原虫抗原的免疫调节作用。由于人体MHC结构的多态性,对疟原虫抗原的免疫反应不完全由MHC所限制,同时也受MHC外的基因所调节。反复感染疟疾常引起机体的免疫抑制现象,作者分析了免疫抑制的机制。     

恶性疟原虫的抗原变异与var基因家族

恶性疟原虫利用改变暴露于宿主免疫系统的抗原逃避宿主的攻击。在恶性疟原虫7号染色体上发现的基因家族var编码了恶性疟原虫红细胞膜蛋白1(Pf—EMP1),并参与调节受染红细胞的抗原变异和与血管内皮细胞的胞间粘附。认识并深入研究var基因有助于人们进一步了解恶性疟原虫抗原变异机制,研制具有针对性的抗疟疫苗,最终达到控制疟疾的目的。     

疟原虫感染的免疫应答及效应分子的研究进展

疟原虫生活史复杂,各期的抗原成分多样化,随着恶性疟原虫抗药性不断扩大,目前仍无特效药物和疫苗预防感染。对恶性疟原虫感染引起的免疫应答的深入研究,提供了大量关于免疫在疟原虫感染中的作用信息。并且随着基因组测序计划的完成,有助于认识疟原虫不同期生长发育的分子机制,为疟疾的预防控制提供理论依据。     

间日疟原虫入侵红细胞的相关蛋白研究进展

基于疟原虫入侵途径的多样化以及红细胞表面受体的多样性,间日疟原虫入侵红细胞的过程呈现高度的种特异性。寻找参与疟原虫入侵红细胞的相关分子,探索其入侵机制是疟疾研究领域的重点。间日疟原虫全基因组测序的完成,为从分子水平探索间日疟原虫的入侵机制和疟疾候选疫苗的研究提供了新的研究方法。本文通过达菲结合蛋白、网织红细胞结合蛋白、间日疟原虫色氨酸富集抗原等几种重要的入侵蛋白,对间日疟原虫入侵红细胞分子机制的研究进展作一综述。     

约氏疟原虫与伯氏疟原虫侵入期抗原的初步研究

目的　用针对鼠约氏疟原虫(Plasmodiumyoelii)侵入期的8种单克隆抗体,对约氏疟原虫和伯氏疟原虫(P.berghei)侵入期即动合子、裂殖子和子孢子棒状体和表面抗原检测分析。　方法　间接免疫荧光实验(IFA)对各侵入期抗原进行亚细胞结构定位,SDSPAGE及Western印迹对两种鼠疟原虫的不同侵入期进行抗原组分分析。　结果　经上述两种方法检测发现,顶端复合体抗原成分复杂,约氏疟原虫和伯氏疟原虫的棒状体有共同的抗原表位,约氏疟原虫的动合子与其自身的裂殖子有类似成分,也有各自独特的抗原。两种鼠疟原虫动合子抗原有类似成分。约氏疟原虫的子孢子具有与裂殖子、动合子不同的抗原成分。　结论　疟原虫侵入期棒状体和表面抗原在同一虫种的不同侵入期和不同虫种中有共同的抗原表位,也有各自的独特组分。     

恶性疟原虫的抗原变异与var基因家族

恶性疟原虫利用改变暴露于宿主免疫系统的抗原逃避宿主的攻击。在恶性疟原虫７号染色体上发现的基因家族ｖａｒ编码了恶性疟原虫红细胞膜蛋白１（Ｐｆ－ＥＭＰ１），并参与调节受染红细胞的抗原变异和与血管内皮细胞的胞间粘附。认识并深入研究ｖａｒ基因有助于人们进一步了解恶性疟原虫抗原变异机制，研制具有针对性的抗疟疫苗，最终达到控制疟疾的目的。     

啮齿动物约氏疟原虫期特异性和种特异性的单克隆抗体及交叉反应抗原

疟原虫感染可产生抗疟原虫抗体,但诱导这些抗体应答的抗原数量和位置以及其特征仍不清楚。过去在识别和分离个别疟原虫抗原以及评定其对免疫的产生和抑制作用亦很困难。而现今应用杂交瘤技术使有可能产生大量对单一疟原虫抗原决定簇的单克隆抗体,并用以识别和最终纯化疟原虫抗原。本文报道了用18种杂交瘤细胞系研究啮齿动物约氏疟原虫的种特异性,期特异性以及血清交叉反应抗原的情况。     

恶性疟原虫非编码RNA的研究进展

疟疾依然是世界上严重危害人类健康的3大传染病之一。在5种导致人类疟疾的疟原虫中,恶性疟原虫(Plasmodium falciparum)毒力最强、致死率最高。研究表明,红内期恶性疟原虫能够逃避宿主免疫系统,与其抗原编码基因的互斥性表达密不可分,这依赖于其发育过程具有精密的基因表达调控机制。目前对其基因表达调控机制研究尚不够深入。已有研究发现,恶性疟原虫中具有丰富的非编码RNA(non-coding RNA,ncRNA),其中一部分ncRNA已被证实在恶性疟原虫生长发育和致病过程相关的基因表达调控中发挥着重要作用。本文就近年恶性疟原虫相关ncRNA的功能研究进展进行综述,以加深对疟原虫基因表达调控机制的理解,从而为疟原虫致病的分子机制研究提供理论基础。     

疟原虫感染的免疫应答及效应分子的研究进展

疟原虫生活史复杂，各期的抗原成分多样化，随着恶性疟原虫抗药性不断扩大，目前仍无特效药物和疫苗预防感染。对恶性疟原虫感染引起的免疫应答的深入研究，提供了大量关于免疫在疟原虫感染中的作用信息。并且随着基因组测序计划的完成，有助于认识疟原虫不同期生长发育的分子机制，为疟疾的预防控制提供理论依据。     

疟原虫抗原成分研究进展

本文综述疟原虫的子孢子抗原(CS蛋白)和红内期抗原(TRAP、GBP、RESA、PMMSA、P(?)HSP-70、S抗原、FIRA等)的主要结构特点及其对人体免疫系统的影响,并对疟原虫入侵肝细胞、红细胞的机制作了简要介绍。从这些已知结构的抗原分子中寻找出能诱导宿主保护性免疫力的表位,将这些表位重组制成了杂交多肽抗原,通过动物和人体的初步实验证实这些杂交多肽抗原是完全有效的。     

Activation by malaria antigens renders mononuclear cells susceptible to HIV infection and re-activates replication of endogenous HIV in cells from HIV-infected adults

We have tested the hypothesis that activation of T cells by exposure to malaria antigens facilitates both de novo HIV infection and viral reactivation and replication. PBMC from malaria-naive HIV-uninfected European donors could be productively infected with HIV following in vitro stimulation with a lysate of Plasmodium falciparum schizonts and PBMC from malaria-naive and malaria-exposed (semi-immune) HIV-positive adults were induced to produce higher levels of virus after stimulation with the same malaria extract. These findings suggest that effective malaria control measures might con-tribute to reducing the spread of HIV and extending the life span of HIV-infected individuals living in malaria endemic areas.     

Acquisition of antibody isotypes against Plasmodium falciparum blood stage antigens in a birth cohort

Information on the period during which infants lose their maternally derived antibodies to malaria and begin to acquire naturally their own immune responses against parasite antigens is crucial for understanding when malaria vaccines may be best administered. This study investigated the rates of decline and acquisition of serum antibody isotypes IgG1, IgG2, IgG3, IgG4, IgM and IgA to Plasmodium falciparum antigens apical membrane antigen (AMA1), merozoite surface proteins (MSP1‐19, MSP2 and MSP3) in a birth cohort of 53 children living in an urban area in the Gambia, followed over the first 3 years of life (sampled at birth, 4, 9, 18 and 36 months). Antigen‐specific maternally transferred antibody isotypes of all IgG subclasses were detected at birth and were almost totally depleted by 4 months of age. Acquisition of specific antibody isotypes to the antigens began with IgM, followed by IgG1 and IgA. Against the MSP2 antigen, IgG1 but not IgG3 responses were observed in the children, in contrast with the maternally derived antibodies to this antigen that were mostly IgG3. This confirms that IgG subclass responses to MSP2 are strongly dependent on age or previous malaria experience, polarized towards IgG1 early in life and to IgG3 in older exposed individuals.     

Malaria vaccines: where are we and where are we going?

Malaria is still killing over one million people each year and its incidence is increasing. The need for an effective vaccine is greater than ever. A major difficulty with vaccine research is that the malaria parasite presents thousands of antigens to the human immune system that vary throughout its life cycle. Identifying those that may prove to be vaccine targets is complicated and time consuming. Most vaccines are targeted at individual stages of the malaria life cycle, although it is likely that only the development of a multistage vaccine will offer complete protection to both visitors to, and residents of, a malaria-endemic area. With the development of a successful vaccine other issues such as cost, distribution, education, and compliance will have to be addressed. This review describes some of the current vaccine candidates for immunising against malaria.     

The effects of varying exposure to malaria transmission on development of antimalarial antibody responses in preschool children. XVI. Asembo Bay Cohort Project

In areas of intense malaria transmission, malaria morbidity and mortality is highest in children 3-18 months old. Interventions that reduce malaria exposure early in life reduce morbidity but may also delay development of clinical immunity. We assessed the relationship between intensity of malaria exposure and development of antibody responses. Thirty-nine children were monitored monthly, from birth to > or =2.5 years old (1238 observations), and were divided into 3 exposure categories, on the basis of parasitemic episodes or entomological data. Children with low exposure during the first 2 years of life had higher subsequent levels of antibody to merozoite surface protein-1(19-kDa) (a marker of blood-stage responses) by months 24-35 (P<.05). This inverse relationship decreased as children aged. There was no consistent relationship between exposure early in life and subsequent levels of antibody to circumsporozoite protein (a marker of sporozoite-stage responses). These data suggest that, in areas of intense malaria transmission, during the first 3 years of life, interventions that either reduce the number of asexual parasitemic episodes or lower entomological exposure do not delay the development of antibody responses to blood-stage malarial antigens.     

Enhanced acquired antibodies to a chimeric Plasmodium falciparum antigen; UB05‐09 is associated with protective immunity against malaria

It has been shown that covalently linking two antigens could enhance the immunogenicity of the chimeric construct. To prioritize such a chimera for malaria vaccine development, it is necessary to demonstrate that naturally acquired antibodies against the chimera are associated with protection from malaria. Here, we probe the ability of a chimeric construct of UB05 and UB09 antigens (UB05‐09) to better differentiate between acquired immune protection and susceptibility to malaria. In a cross‐sectional study, recombinant UB05‐09 chimera and the constituent antigens were used to probe for specific antibodies in the plasma from children and adults resident in a malaria‐endemic zone, using the enzyme‐linked immunosorbent assay (ELISA). Anti‐UB05‐09 antibody levels doubled that of its constituent antigens, UB09 and UB05, and this correlated with protection against malaria. The presence of enhanced UB05‐09‐specific antibody correlated with the absence of fever and parasitaemia, which are the main symptoms of malaria infection. The chimera is more effective in detecting and distinguishing acquired protective immunity against malaria than any of its constituents taken alone. Online B‐cell epitope prediction tools confirmed the presence of B‐cell epitopes in the study antigens. UB05‐09 chimera is a marker of protective immunity against malaria that needs to be studied further.     

Suppression of in–vitro lymphoproliferative responses in acute malaria patients can be partially reversed by indomethacin

In-vitro lymphoproliferative responses to malaria antigens are suppressed in patients with acute Plasmodium falciparum infection. Studies with other parasitic diseases have suggested that monocyte/macrophage-derived prostaglandins may be responsible for immunosuppression. Since acute malaria infection is characteristically associated with fever it is likely that prostaglandin E production will also be enhanced in these patients. In this study, indomethacin, a cyclooxygenase inhibitor which blocks the synthesis of prostaglandins, was added to the culture medium during assays of lymphoproliferative responses to malaria antigens and other soluble proteins. Responses to several antigens were enhanced in the presence of indomethacin, indicating that prostaglandins may have a generalized immunosuppressive role in malaria-infected individuals. However, responses to malaria antigens were particularly enhanced by indomethacin, suggesting that malaria-specific T-cells are especially sensitive to the effects of prostaglandin, possibly due to prior activation in vivo by circulating malaria antigens.     

Boosting antibody responses to Plasmodium falciparum merozoite antigens in children with highly seasonal exposure to infection

Longitudinal cohort studies are important to describe the dynamics of naturally acquired antibody response profiles to defined Plasmodium falciparum malaria antigens relative to clinical malaria episodes. In children under 7 years of age in The Gambia, serum IgG responses were measured to P. falciparum merozoite antigens AMA1, EBA175, MSP1₁₉, MSP2 and crude schizont extract, over a 10‐month period. Persistence of antibody responses was measured in 152 children during the dry season when there was virtually no malaria transmission, and 103 children were monitored for new episodes of clinical malaria during the subsequent wet season when transmission occurred. Children who experienced clinical malaria had lower antibody levels at the start of the study than those who remained free from malaria. Associations between dry season antibody persistence and subsequent wet season antibody levels suggested robust immunological memory responses. Mean antibody levels to all antigens were elevated by the end of the wet season in children who experienced clinical malaria; each of these children had a boosted antibody response to at least one antigen. In all children, antibody avidities were lower against MSP2 than other antigens, a difference that did not change throughout the study period or in relation to clinical malaria episodes.     

Do antibody responses to malaria vaccine candidates influenced by the level of malaria transmission protect from malaria?

Objectives To examine whether the humoural response to malaria vaccine candidate antigens, Plasmodium falciparum [circumsporozoite repetitive sequence (NANP)₅ GLURP fragments (R0 and R2) and MSP3] varies with the level of malaria transmission and to determine whether the antibodies (IgG) present at the beginning of the malaria transmission season protect against clinical malaria. Methods Cross‐sectional surveys were conducted to measure antibody response before, at the peak and at the end of the transmission season in children aged 6 months to 10 years in two villages with different levels of malaria transmission. A cohort study was performed to estimate the incidence of clinical malaria. Results Antibodies to these antigens showed different seasonal patterns. IgG concentrations to any of the four antigens were higher in the village with high entomological inoculation rate. Multivariate analysis of combined data from the two villages indicated that children who were classified as responders to the selected antigens were at lower risk of clinical malaria than children classified as non‐responders [(NANP)₅ (incidence rate ratio (IRR) = 0.65, 95% CI: 0.46–0.92; P = 0.016), R0 (IRR = 0.69, 95% CI: 0.48–0.97; P = 0.032), R2 (IRR = 0.73, 95% CI: 0.50–1.06; P = 0.09), MSP3 (IRR = 0.52, 95% CI: 0.32–0.85; P = 0.009)]. Fitting a model with all four antibody responses showed that MSP3 looked the best malaria vaccine candidate (IRR = 0.63; 95% CI: 0.38–1.05; P = 0.08). Conclusion Antibody levels to the four antigens are affected by the intensity of malaria transmission and associated with protection against clinical malaria. It is worthwhile investing in the development of these antigens as potential malaria vaccine candidates.     

Multiple cross-reactivities amongst antigens of Plasmodium falciparum impair the development of protective immunity against malaria

The majority of protein antigens of the malaria parasite Plasmodium falciparum contain short sequences that are extensively repeated in tandem arrays. Some antigens contain a single block of repeats whereas in other antigens there may be two or more blocks of related repeats. The repetitive sequences in an individual antigen may be highly conserved but more usually there is some degeneracy which occasionally is extensive. The repetitive sequences encode immunodominant epitopes to which much of the antibody response in malaria is directed. Recently, we have found that there are extensive cross-reactions amongst the epitopes encoded by related repetitive sequences. These cross-reactions may involve different blocks of repeats in the one antigen or repetitive sequences in different antigens. It is proposed that these cross-reactions interfere with the normal maturation of a high affinity antibody response in malaria by causing an abnormally high proportion of somatically-mutated B cells to be preserved during clonal expansion.     

Differential T‐cell responses to a chimeric Plasmodium falciparum antigen; UB05‐09, correlates with acquired immunity to malaria

The development of a sterilizing and cost‐effective vaccine against malaria remains a major problem despite recent advances. In this study, it is demonstrated that two antigens of P. falciparum UB05, UB09 and their chimera UB05‐09 can serve as protective immunity markers by eliciting higher T‐cell responses in malaria semi‐immune subjects (SIS) than in frequently sick subjects (FSS) and could be used to distinguish these two groups. UB05, UB09 and UB05‐09 were cloned, expressed in E. coli, purified and used to stimulate PBMCs isolated from 63 subjects in a malaria endemic area, for IFN‐γ production, which was measured by the ELISpot assay. The polymorphism of UB09 gene in the malaria infected population was also studied by PCR/sequencing of the gene in P. falciparum field isolates. All three antigens were preferentially recognized by PBMCs from SIS. IFN‐γ production induced by these antigens correlated with the absence of fever and parasitaemia. UB09 was shown to be relatively well‐conserved in nature. It is concluded that UB05, UB09 and the chimera UB05‐09 posses T‐cell epitopes that are associated with protection against malaria and could thus be used to distinguish SIS from FSS eventhough acute infection with malaria has been shown to reduce cytokine production in some studies. Further investigations of these antigens as potential diagnostic and/or vaccine candidates for malaria are indicated.