Absence of Erythrocyte Sequestration and Lack of Multicopy Gene Family Expression in Plasmodium falciparum from a Splenectomized Malaria Patient

Abstract

BackgroundTo avoid spleen-dependent killing mechanisms parasite-infected erythrocytes (IE) of Plasmodium falciparum malaria patients have the capacity to bind to endothelial receptors. This binding also known as sequestration, is mediated by parasite proteins, which are targeted to the erythrocyte surface. Candidate proteins are those encoded by P. falciparum multicopy gene families, such as var, rif, stevor or PfMC-2TM. However, a direct in vivo proof of IE sequestration and expression of multicopy gene families is still lacking. Here, we report on the analysis of IE from a black African immigrant, who received the diagnosis of a malignant lymphoproliferative disorder and subsequently underwent splenectomy. Three weeks after surgery, the patient experienced clinical falciparum malaria with high parasitemia and circulating developmental parasite stages usually sequestered to the vascular endothelium such as late trophozoites, schizonts or immature gametocytes.Methodology/Principal FindingsInitially, when isolated from the patient, the infected erythrocytes were incapable to bind to various endothelial receptors in vitro. Moreover, the parasites failed to express the multicopy gene families var, A-type rif and stevor but expression of B-type rif and PfMC-2TM genes were detected. In the course of in vitro cultivation, the parasites started to express all investigated multicopy gene families and concomitantly developed the ability to adhere to endothelial receptors such as CD36 and ICAM-1, respectively.Conclusion/SignificanceThis case strongly supports the hypothesis that parasite surface proteins such as PfEMP1, A-type RIFIN or STEVOR are involved in interactions of infected erythrocytes with endothelial receptors mediating sequestration of mature asexual and immature sexual stages of P. falciparum. In contrast, multicopy gene families coding for B-type RIFIN and PfMC-2TM proteins may not be involved in sequestration, as these genes were transcribed in infected but not sequestered erythrocytes.

The pathogenesis of Plasmodium falciparum malaria in humans: insights from splenic physiology

“…This hypothesis is consistent with important deposits of malaria pigment in the spleen of the studied patient. 39 The existence of such an intrasplenic cryptic cycle fits well with the major features of hyper-reactive malaria splenomegaly namely, massive splenomegaly, absence of fever, absent or rare parasites on blood smears, high levels of specific antibodies, very high levels of total immunoglobulin M, and positive outcome after sustained antimalarial therapy. 68 When the spleen is removed in such patients, mechanical retention, the only powerful mechanism able to clear adhesin-less variants, disappears, giving rise to a rapid increase of parasite loads and an acute clinical attack (Figure 3Dii).…”

“…The analysis of published data (summarized in Table 1) shows that among 94 splenectomized patients with P falciparum infection (corresponding to 17 articles 38,39,51-65 ), 3 were experiencing their first malaria attack ("naïve") and 22 had few previous attacks or undocumented history (and "possibly immune"), and 69 had been living in an endemic area for more than 8 years before splenectomy ("immune"), [63][64][65] and 3 experienced a switch from chronic carriage to symptomatic attack within 3-8 weeks after splenectomy. 39,62 Splenectomy also affects the clearance of iRBCs after antimalarial treatment. [58][59][60][61] Data and interpretation on each of these different situations are analyzed separately in Table 1 and below ( Figure 3).…”

“…39,62 In the single patient in whom this has been explored to date, there was no expression of genes for surface adhesins, and cytoadherence of iRBCs during the first cycle after sample collection, but both were progressively restored in culture. 39 This was interpreted as reflecting the positive selection for rare variants in immune patients with a normal spleen function (Figure 3Ci), which if P falciparum carriage is maintained long enough without reinfection ultimately leads to the selection of adhesin-less variants (Figure 3Di) that possibly escape clearance and destruction by the spleen. 39 How can such adhesin-less mature forms multiply without being cleared by the spleen?…”

“…39 This was interpreted as reflecting the positive selection for rare variants in immune patients with a normal spleen function (Figure 3Ci), which if P falciparum carriage is maintained long enough without reinfection ultimately leads to the selection of adhesin-less variants (Figure 3Di) that possibly escape clearance and destruction by the spleen. 39 How can such adhesin-less mature forms multiply without being cleared by the spleen? This may result either from their mechanical retention in the microvasculature or from some low-grade intrasplenic development with reinvasion in the RBC-rich cords, giving rise to a new, low-grade wave of parasites (Figure 3Di).…”

Pathogenic mechanisms in Plasmodium falciparum malaria

“…El bazo remueve los glóbulos rojos parasitados por su menor capacidad de deformación y los eritrocitos sensibilizados con inmunoglobulinas durante la infección por P. falciparum; por ello, el secuestro de glóbulos rojos parasitados con formas maduras permite evadir la circulación esplénica y la destrucción de estas células alteradas. El secuestro de glóbulos rojos parasitados parece ser un mecanismo relacionado con la presencia del bazo, porque se ha descrito que parásitos obtenidos de pacientes con esplenectomía no producen citoadherencia, ya que se observan estadios maduros circulantes e incapacidad de unirse a receptores endoteliales in vitro (98,99). A pesar de que la destrucción esplénica de glóbulos rojos parasitados contribuye a la anemia, el bazo parece jugar un papel benéfico contra las complicaciones; en los estudios de perfusión ex vivo de bazo humano se demuestran que un 10 % de los anillos quedan retenidos en cada paso por el bazo, lo que indica que este órgano de filtración puede controlar la biomasa de parásitos que puede secuestrarse (100).…”