Spleen-Dependent Regulation of Antigenic Variation in Malaria Parasites: Plasmodium knowlesi SICAvar Expression Profiles in Splenic and Asplenic Hosts

Abstract

BackgroundAntigenic variation by malaria parasites was first described in Plasmodium knowlesi, which infects humans and macaque monkeys, and subsequently in P. falciparum, the most virulent human parasite. The schizont-infected cell agglutination (SICA) variant proteins encoded by the SICAvar multigene family in P. knowlesi, and Erythrocyte Membrane Protein-1 (EMP-1) antigens encoded by the var multigene family in P. falciparum, are expressed at the surface of infected erythrocytes, are associated with virulence, and serve as determinants of naturally acquired immunity. A parental P. knowlesi clone, Pk1(A+), and a related progeny clone, Pk1(B+)1+, derived by an in vivo induced variant antigen switch, were defined by the expression of distinct SICA variant protein doublets of 210/190 and 205/200 kDa, respectively. Passage of SICA[+] infected erythrocytes through splenectomized rhesus monkeys results in the SICA[-] phenotype, defined by the lack of surface expression and agglutination with variant specific antisera.Principal FindingsWe have investigated SICAvar RNA and protein expression in Pk1(A+), Pk1(B+)1+, and SICA[-] parasites. The Pk1(A+) and Pk1(B+)1+ parasites express different distinct SICAvar transcript and protein repertoires. By comparison, SICA[-] parasites are characterized by a vast reduction in SICAvar RNA expression, the lack of full-length SICAvar transcript signals on northern blots, and correspondingly, the absence of any SICA protein detected by mass spectrometry.SignificanceSICA protein expression may be under transcriptional as well as post-transcriptional control, and we show for the first time that the spleen, an organ central to blood-stage immunity in malaria, exerts an influence on these processes. Furthermore, proteomics has enabled the first in-depth characterization of SICA[+] protein phenotypes and we show that the in vivo switch from Pk1(A+) to Pk1(B+)1+ parasites resulted in a complete change in SICA profiles. These results emphasize the importance of studying antigenic variation in the context of the host environment.

Plasmodium vivax trophozoite-stage proteomes

“…This is true with regard to understanding the intra-eythrocytic development cycle (IDC) of the parasites growing and multiplying within RBCs over the course of their 24–72 h (depending on the species), but also in terms of pathogenesis and immune evasion strategies that are the result of antigenic variation mechanisms [20]. We have shown that the presence of SICAvar transcripts alone is not necessarily an indicator of the subsequent expression of the encoded SICA proteins in P. knowlesi and that the spleen plays a role in transcriptional or post-transcriptional regulatory processes [21]. Whether similar tactics govern certain gene and protein expression mechanisms in P. vivax remains unknown.…”

“…These biological differences include the expression in P. vivax (and P. cynomolgi [16]) of members of a multigene family called vir, which encodes several hundred small presumptive variant antigen proteins with multiple predicted localizations [17–19]. This is in contrast to the ~60 member var gene family in P. falciparum and the related ~108 member SICAvar family in Plasmodium knowlesi , with each confirmed to encode large variant antigens that become positioned at the surface of the infected RBCs and undergo switching events in the course of an immune response [20,21]. …”

From within host dynamics to the epidemiology of infectious disease: Scientific overview and challenges

“…Because the parasite is threatened by the immune system, it has evolved escape strategies, including the intriguing use of antigenic variation mechanisms [89; 90; 91]. Most notably, P. falciparum and P. knowlesi have large multigene families, respectively, called the var and SICAvar gene families with about 60 ( var ) and 100 ( SICAvar ) members, that encode related variant proteins with different antigenic phenotypes [92; 93; 94; 95]. This topic is discussed again further below in relation to modeling within and between host-infection dynamics.…”

Conserved associations between G-quadruplex-forming DNA motifs and virulence gene families in malaria parasites

“…Effective antigenic variation over long periods of time requires highly regulated and mutually exclusive gene expression, and this has indeed been well characterized in the case of the var genes [12]. Other families, including sicavar [13] and pir [14], appear to have similar dynamics, albeit with less tight mutually-exclusive regulation. In the case of P. falciparum, var genes are regulated by epigenetic silencing and expression switching [12], and additional antigenic diversity is generated within the~60-member gene family via frequent recombination during both mitosis and meiosis [15,16].…”