Association between the tumor necrosis factor locus and the clinical outcome of Leishmania chagasi infection

Abstract

A periurban outbreak of visceral leishmaniasis (VL) caused by the protozoan Leishmania chagasi is ongoing outside Natal, northeast Brazil. Manifestations range from asymptomatic infection to disseminated visceral disease. Literature reports suggest that both genetic and environmental factors influence the outcome of infection. Due to the association of the tumor necrosis factor (TNF) locus with other infectious diseases, we examined whether polymorphic alleles at this locus are associated with the outcome of L. chagasi infection. Neighborhoods with ongoing transmission were identified through patients admitted to local hospitals. Altogether, 1,024 individuals from 183 families were classified with the following disease phenotypes: (i) symptomatic VL, (ii) asymptomatic infection (positive delayed-type hypersensitivity [DTH+]), or (iii) no evidence of infection (DTH-). Genotypes were determined at a microsatellite marker (MSM) upstream of the TNFB gene encoding TNF-beta and at a restriction fragment length polymorphism (RFLP) at position -307 in the promoter of the TNFA gene encoding TNF-alpha. Analyses showed that the distribution of TNFA RFLP alleles (TNF1 and TNF2) and the TNF MSM alleles (TNFa1 to TNFa15) differed between individuals with VL and those with DTH+ phenotypes. TNF1 was transmitted more frequently than expected from heterozygous parents to DTH+ offspring (P = 0.0006), and haplotypes containing TNF2 were associated with symptomatic VL (P = 0.0265, transmission disequilibrium test). Resting serum TNF-alpha levels were higher in TNF1/2 heterozygotes than in TNF1/1 homozygotes (P < 0.05). These data led us to hypothesize that an individual's genotype at the TNF locus may be associated with whether he or she develops asymptomatic or symptomatic disease after L. chagasi infection. The results preliminarily suggest that this may be the case, and follow-up with larger populations is needed for verification.

FIG. 1.

Map of the human TNF locus and HLA loci based upon the studies of Wilson et al. (61) and Zanelli et al. (64). The drawing indicates the relative positions of the TNFA and TNFB genes encoding TNF-α and TNF-β, respectively, with respect to HLA regions on human chromosome 6p. Rectangles indicate genes, and horizontal arrows above genes show the direction of transcription. Vertical arrows indicate the positions of markers used in this study. These include the polymorphic TNF MSM repeat upstream of TNFB; the RFLP marker for TNF1 and TNF2 alleles in the TNFA promoter, located at position −307 (called −308 in the literature) with respect to transcription; and three MSMs in flanking HLA regions. HSP70-2/1 indicates the location of the HSP70-2 (centromeric) and HSP70-1 (telomeric) genes. Distances between regions, marked with double-headed arrows, are indicated in kilobases or megabases. The figure is not drawn to scale.