Convergent evolution in European and Rroma populations reveals pressure exerted by plague on Toll-like receptors

Abstract

Recent historical periods in Europe have been characterized by severe epidemic events such as plague, smallpox, or influenza that shaped the immune system of modern populations. This study aims to identify signals of convergent evolution of the immune system, based on the peculiar demographic history in which two populations with different genetic ancestry, Europeans and Rroma (Gypsies), have lived in the same geographic area and have been exposed to similar environments, including infections, during the last millennium. We identified several genes under evolutionary pressure in European/Romanian and Rroma/Gipsy populations, but not in a Northwest Indian population, the geographic origin of the Rroma. Genes in the immune system were highly represented among those under strong evolutionary pressures in Europeans, and infections are likely to have played an important role. For example, Toll-like receptor 1 (TLR1)/TLR6/TLR10 gene cluster showed a strong signal of adaptive selection. Their gene products are functional receptors for Yersinia pestis, the agent of plague, as shown by overexpression studies showing induction of proinflammatory cytokines such as TNF, IL-1β, and IL-6 as one possible infection that may have exerted evolutionary pressures. Immunogenetic analysis showed that TLR1, TLR6, and TLR10 single-nucleotide polymorphisms modulate Y. pestis-induced cytokine responses. Other infections may also have played an important role. Thus, reconstruction of evolutionary history of European populations has identified several immune pathways, among them TLR1/TLR6/TLR10, as being shaped by convergent evolution in two human populations with different origins under the same infectious environment.

Keywords: immunity; migration; pandemics; pattern recognition receptors.

Fig. 1.

Geographic origin of the three populations studied. (A) European/Romanians and Rroma/Gipsy share the same location, even if the origin of the latter is in North India. (B) Plot of the populations under analysis according to the coordinates to the two main eigenvectors of smartpca (Eigensoft) analysis, in which each dot represents an individual. Individuals within the circles and the same color have been considered for the study; those of different colors represent false population allocation and those intermediate represent admixed individuals. ROM, nongypsy Romanians; INDI, individuals from North India; GYP, Rroma/Gypsies living in Romania.

Fig. 2.

Manhattan plot of results of selection tests in Rroma, Romanians, and Indians using TreeSelect statistic (A) and XP-CLR statistic (B). Chromosomes ordered from chromosome 1 to chromosome 22.

Fig. 3.

The role of TLR10 for the recognition of Y. pestis and Y. pseudotuberculosis. (A) HEK293 transiently transfected with TLR2, TLR10, or TLR2/10, and stimulated with 1 × 10⁵ heat-inactivated Y. pestis or Y. pseudotuberculosis, respectively. Bars represent the means ± SEM of at least three separate experiments. (B) PBMCs stimulated with Y. pestis or Y. pseudotuberculosis per mL. n = 6; means ± SEM; *P = 0.05, **P = 0.01. (C) TNF-α production after PBMCs stimulated with Y. pestis or Y. pseudotuberculosis in the presence or absence of 10 µg/mL antibody. (D) IL-1β production after 24 h of stimulation. Means ± SEM; *P = 0.05, **P = 0.01. The data shown are from three independent experiments each performed in duplicate.

Fig. 4.

Functional consequences of human TLR1/TLR6/TLR10 SNPs for Y. pestis–stimulated cytokine production. PBMCs from healthy volunteers stimulated with different stimuli, including Y. pestis (1 × 10⁵/mL). Volunteers were separated into three groups: one group did not display the SNP in either TLR1 (A/B), TLR6 (C/D), or TLR10 (E/F; wt, wild-type); one group was heterozygous for the polymorphism (He); and one group was homozygous (Ho). Data are means ± SEM. *P = 0.05, **P = 0.01, ***P = 0.001.