Genes influencing resistance to Coccidioides immitis and the interleukin-10 response map to chromosomes 4 and 6 in mice

Abstract

Coccidioidomycosis is a fungal infection that is endemic in the southwestern United States. Infection is more severe in blacks and Filipinos, which suggests that there is a genetic basis for susceptibility to this infection in humans. We found that there is also a difference in resistance to Coccidioides immitis infection among inbred mouse strains: B6 mice are susceptible, while DBA/2 mice are resistant (T. N. Kirkland and J. Fierer, Infect. Immun. 40:912-916, 1983). In this paper we report the results of our efforts to map the genes responsible for resistance to this infection in mice. Mice were infected by intraperitoneal inoculation, and 15 days later the numbers of viable fungi in their lungs and spleens were enumerated. We also determined the amounts of interleukin-10 mRNA made in the infected lungs. These three phenotypes were mapped as quantitative traits by using the 26 available lines of recombinant inbred mice derived from a cross between B6 and DBA/2 mice. The best associations were those between the regions near the Lv locus on chromosome 4 and the Tnfr1 locus on chromosome 6. We then infected backcross mice [(B6 x DBA/2) x B6] and confirmed these associations; 14 of 16 (87%) mice that were heterozygous at both Lv and Tnfr1 were resistant to infection, whereas only 4 of 16 (25%) mice that were homozygous B6 at both loci were resistant. These are the first genetic loci to be associated with susceptibility to C. immitis, but there may be additional genes involved in murine resistance to this infection.

FIG. 1

Comparison of the time courses of C. immitis infection in resistant DBA/2 mice (open circles) and susceptible B6 mice (solid triangles), as measured by quantitative colony counts in their lungs. Each point represents a single mouse. Dead mice were assigned 10⁷ CFU/lung, the number recovered from the most heavily infected live mouse. The dead mice are indicated with a “d.” The one remaining B6 mouse died on day 16 and was necropsied immediately.

FIG. 2

Distribution of values for CFU per lung on day 15 after infection with ∼500 arthroconidia injected i.p. Open bars, values for DBA/2 mice; solid bars, values for B6 mice. For simplicity, mice with colony counts between the 0.5 log markers were grouped together into the next category.

FIG. 3

Variation in severity of C. immitis infection among 26 B×D RI lines. Each point is the median value for CFU per lung for each RI line. There were at least 10 mice in each group, except for B×D group 13, which had only 4 mice because of limited availability of this line. Values for the control B6 and DBA/2 mice are shown for comparison.

FIG. 4

Correlation between median values for CFU and IL-10 mRNA levels in 26 B×D RI lines. IL-10 mRNA was measured by a quantitative reverse transcription-PCR assay, pooling equal amounts of RNA from four mice in each group. Each point is a single RI line (r = 0.77).