Parasite virulence in coinfected hosts: the importance of infection order

When hosts are simultaneously infected by different pathogens, the severity of the disease might be exacerbated compared to hosts harboring single infections. The reasons underlying these changes in parasite virulence are manifold. Here, we investigated the importance for the infection outcome of order and timing of infection. We used a model of rodent coinfection between two parasites that do not compete for common resources, an intestinal nematode ( Heligmosomoides polygyrus ) and an apicomplexan protozoan ( Plasmodium yoelii ). During single infections, H. polygyrus induced only mild disease symptoms. Plasmodium produced a substantial reduction in the number of red blood cells but all mice recovered from the infection. A different picture emerged in coinfected hosts. Heligmosomoides maintained a profile of mostly asymptomatic infection whatever the order of the infection (first or second). On the contrary, P. yoelii incurred substantially higher costs in hosts that had been previously infected with H. polygyrus (independently of the timing of infection). We then investigated the possible reasons underlying the increase of Plasmodium virulence in coinfected hosts. We found that coinfected hosts were less able to control Plasmodium multiplication and to recover from infection-induced anemia. Coinfected hosts had similar levels of erythropoietin and similar renewal of lost red blood cells, resulting in decreased tolerance to Plasmodium infection. Experimental administration of erythropoietin in coinfected hosts, partially decreased the severity of disease symptoms and improved tolerance. The detoxification of free heme released during the lysis of red blood cells, and the expression of Th1 and anti-inflammatory cytokine genes were also similar between coinfected and single infected hosts. However, coinfected mice had higher proportions of regulatory T cells expressing the CTLA-4 immune checkpoint, suggesting an enhanced immunosuppressive activity of Tregs. Plasmodium infection also induced the exhaustion of CD8 ⁺ T cells, as coinfected mice had higher proportions of both PD-1 ⁺ and LAG-3 ⁺ CD8 ⁺ T cells, and an increase in the CD4 ⁺ /CD8 ⁺ ratio. Overall, these results stress the importance of the order of infection as a major determinant of disease severity in coinfected hosts. We discuss the possible epidemiological and evolutionary consequences of these results.