Apicomplexa: Malaria
3.4 Adaptive immunity
3.3.2.4 ␥␦T cells
Infection with either P. falciparum or P. vivax results in an expansion of ␥␦
T cells that are thought to recognise malaria-derived non-peptidic phospho- antigens. Activation of ␥␦ T cells in malaria infection requires exogenous cytokine stimulation from other cells of the immune system. However, when activated, these cells produce IFN-␥ and perform cytotoxic actions on infected RBCs.
3.3.2.5 Dendritic cells
Myeloid (but not plasmacytoid) DCs are generally believed to play an important role in priming T cells in malaria infection, in turn activating adaptive immune responses against malaria. Uptake of infected RBCs, as in macrophages, can occur via opsonic or non-opsonic routes. Again, non-opsonic uptake of P.
falciparum infected RBC is thought to occur via Pf EMP-1 interactions with CD36. This interaction has been shown to suppress the response of DCs to secondary TLR stimulation, as evidenced by a diminished up-regulation of MHC-II or co-stimulatory molecules CD40, CD80, CD86 in response to lipopolysaccharide (LPS).
In the P. yoelii mouse model of malaria, DCs phagocytose infected RBCs and digest them in acidified mature phagosomes. However, this process does not always appear to result in activation of DCs, which may be dependent on recog- nition of parasite products released at schizogony. Lysed P. yoelii infected RBCs activate DCs via activation of the MyD88, an adaptor protein used by PRRs to activate the transcription of pro-inflammatory cytokines. Some of the products of lysed infected RBCs responsible for this activation are listed in Table 3.1.
In contrast to the P. yoelii model of mouse malaria, intact P. chabaudi-infected RBC can activate mouse DCs, inducing up-regulation of MHC-II and co- stimulatory molecules CD80/CD86. One of the primary splenic DC subsets in the mouse to prime CD4+ T cells towards a Th1 phenotype in P. chabaudi infection are CD8+ DCs. Activation of CD8+ T cells via cross-presented anti- gens on MHC-I in P. berghei infection has also been shown to occur via the CD8+ DC subset. The activation of regulatory DC subsets, such as the CD11clowCD45RBhighDCs in the spleen during P. yoelii infection, may play a role in immunoregulation of malaria infection via the priming and expansion of IL- 10 expressing CD4+ T cells, a T cell subset that is able to reduce immunopathol- ogy in malaria infection.
Antibody-opsonised sporozoites are susceptible to destruction by complement mediated-lysis, in addition to Fc receptor-mediated lysis by NK or NKT cells and phagocytosis by macrophages.
3.4.1.2 T cell immunity to intra-hepatic stages
Sporozoites that escape antibody-based mechanisms are able to infect hepato- cytes. Developing LEFs are susceptible to immune responses mediated by both CD4+ and CD8+ T cells. In the P. yoelii mouse model, immunisation of mice with linear peptides derived from the intra-hepatic stage P. yoelii proteins, sur- face sporozoite protein (PySSP2), or hepatic and erythrocytic stage protein of 17kDa (PyHEP17), generates a CD4+ T cell response that confers solid protec- tive immunity against challenge infection in an IFN-␥-dependent manner.
CD8+ T cells and IFN␥ are indispensable for effective immune responses to the intra-hepatic stages. In vivo depletion of CD8+ T cells completely abrogates protection afforded by injection with sporozoites that are attenuated (and un- able to develop) due either to exposure to radiation or to genetic modification.
Mice that are engineered to be deficient in MHC-I expression (and are there- fore unable to present parasite-derived peptides on infected hepatocytes for targeted destruction by CD8+ T cells) are also not protected from a challenge infection by immunisation of attenuated sporozoites.
The anti-parasitic effect of CD8+ T cells is dependent on IFN-␥, since in vivo depletion of IFN-␥ leads to abrogation of protective anti-sporozoite immu- nity. CD8+ T cells could kill infected hepatocytes directly via targeted release of cytotoxic pore-forming molecules such as perforin and granzymes, leading to necrotic destruction. Alternatively, apoptosis of infected hepatocytes could be induced via Fas-FasL signalling. In both cases, entry of the parasites to the asexual erythrocytic cycle is prevented.
Although infected hepatocytes and/or resident liver antigen-presenting cells can present parasite antigens complexed to MHC-I molecules to prime CD8+
T cells, evidence suggests that dendritic cells (DCs) play an essential role in the priming of T cells against pre-erythrocytic stages. It is not clear where DCs prime CD8+ T cells capable of recognising infected hepatocytes, but the anti- genic specificity of T cells reactive against sporozoites and against infected hepatocytes has some similarity (for example thrombospondin-related anony- mous protein (TRAP) and CSP are antigens common to both stages). Therefore, it is thought that DCs that become primed in the skin, while sporozoites reside in the avascular space (or in the lymph nodes draining the site of sporozoite deposition), may have an important role in activating CD8+ T cells that can subsequently lyse infected hepatocytes containing LEFs.
3.4.2 Immunity to the asexual erythrocytic cycle
3.4.2.1 CD4 T cells
Infected RBCs express parasite-exported antigens on the surface, facilitating recognition, uptake and processing by antigen presenting cells followed by sub- sequent activation of CD4+ T cells. The activated CD4+ T cells secrete a variety
of cytokines and/or parasiticidal molecules that will have either a direct or an indirect effect on parasite killing.
During the acute phase of a malaria infection, the responding CD4+ T cells are predominantly of the Th1 phenotype and are thought to induce cell-mediated parasiticidal mechanisms by phagocytic cells. Subsequently, CD4+ T cells with a Th2 phenotype arise, reflecting the importance of CD4+ T cell help for anti- body production by B cells that eventually limits parasite density. Consistent with a role for CD4+ T cell help for protective antibody production by B cells, mice that are deficient in CD4+ T cells cannot make a sufficient antibody re- sponse to control malaria parasitaemia in rodent models of malaria infection.
3.4.2.2 B cells and antibodies
Antibodies were shown to be a critical component of naturally acquired immu- nity to the blood-stages of malaria when purified IgG was passively transferred from ‘malaria-immune’ adults to patients with clinical malaria, resulting in res- olution of symptoms as well as significant reductions in parasitaemia. Protec- tive antibody against erythrocytic stage parasites can be targeted to the free merozoite surface, the parasite infected erythrocyte, or to gametocytes, as de- scribed in Table 3.2.
Antibodies can function to control parasite density by opsonising infected RBC or free merozoites, thereby facilitating their removal by macrophages and
Table 3.2 Protective mechanisms of antibodies against the erythrocytic stages of malaria.
Antibody target
Example of target
antigens Mechanism of neutralisation Free merozoites MSP-1
MSP-2 MSP3 AMA-1
1. Blockage of parasite attachment onto uninfected red blood cells (invasion) 2. Interference with proteolytic processing of
proteins critical for invasion
3. Prevention of shedding of invasion ligands 4. Opsonisation to facilitate removal by
phagocytic cells Infected RBCs,
gametocytes
Pf EMP-1 1. Prevention of sequestration via adherence to ICAM-1 or VCAM-1, by blocking the adherence domains onPf EMP-1 2. Prevention of gametocyte maturation 3. Complement mediated lysis of infected RBCs 4. Opsonisation to facilitate removal by
phagocytic cells
‘Malaria toxins’
released at schizogony
GPI anchors Neutralisation, thereby prevention ligation of TLR2 and subsequent pathogenic inflammation
Abbreviations: AMA-1- apical membrane antigen-1; ICAM-1- intercellular adhesion molecule; MSP-1- merozoite sur- face protein-1;PfEMP-1-Plasmodium falciparumerythrocyte membrane protein-1; RBC- red blood cell; TLR-2-Toll- like receptor 2; VCAM-1- vascular cellular adhesion molecule.
preventing the reinvasion of merozoites into RBCs. Antibodies can also help to ameliorate pathogenic processes in malaria infection by helping to prevent se- questration of infected RBC or release of ‘malaria toxins’ at schizogony.