The resistance of BALB/cJ mice to Yersinia pestis maps to the major histocompatibility complex of chromosome 17

Abstract

Yersinia pestis, the causative agent of plague, has been well studied at the molecular and genetic levels, but little is known about the role that host genes play in combating this highly lethal pathogen. We challenged several inbred strains of mice with Y. pestis and found that BALB/cJ mice are highly resistant compared to susceptible strains such as C57BL/6J. This resistance was observed only in BALB/cJ mice and not in other BALB/c substrains. Compared to C57BL/6J mice, the BALB/cJ strain exhibited reduced bacterial burden in the spleen and liver early after infection as well as lower levels of serum interleukin-6. These differences were evident 24 h postinfection and became more pronounced with time. Although a significant influx of neutrophils in the spleen and liver was exhibited in both strains, occlusive fibrinous thrombi resulting in necrosis of the surrounding tissue was observed only in C57BL/6J mice. In an effort to identify the gene(s) responsible for resistance, we measured total splenic bacteria in 95 F(2) mice 48 h postinfection and performed quantitative trait locus mapping using 58 microsatellite markers spaced throughout the genome. This analysis revealed a single nonrecessive plague resistance locus, designated prl1 (plague resistance locus 1), which coincides with the major histocompatibility complex of chromosome 17. A second screen of 95 backcrossed mice verified that this locus confers resistance to Y. pestis early in infection. Finally, eighth generation backcrossed mice harboring prl1 were found to maintain resistance in the susceptible C57BL/6J background. These results identify a novel genetic locus in BALB/cJ mice that confers resistance to Y. pestis.

FIG. 1.

BALB/cJ mice, unlike other BALB/c substrains, are uniquely resistant to Y. pestis. (A and B) Groups of 10 C57BL/6J, BALB/cJ, BALB/cByJ, and BALB/cAnNHsd mice were infected with 55 (A) or 1,000 (B) CFU of Y. pestis KIM5 and monitored for survival for 15 days. (C) Groups of 10 C57BL/6J, BALB/cJ, and CB6F1/J mice were infected with 13,000 CFU of Y. pestis and monitored for survival for 15 days.

FIG. 2.

BALB/cJ mice display lower bacterial burden early after infection than the susceptible C57BL/6J strain. Groups of 12 C57BL/6J, BALB/cJ, and CB6F1/J mice were infected with 2,000 CFU of Y. pestis KIM5. At 24 and 48 h postinfection, six mice from each group were sacrificed, and bacterial titers in the spleen (A) and liver (B) of the animals were determined. Horizontal lines represent the mean for each group. Asterisks indicate significant differences between groups as determined by a Student's t test where P is <0.05.

FIG. 3.

Linkage analysis and interval mapping reveals a single significant QTL on chromosome 17. (A) Ninety-five F₂ (CB6/F1J × CB6/F1J) mice were infected with 3,500 CFU of Y. pestis KIM5, and 48 h postinfection total splenic CFU was determined. LRS scores were generated using Map Manager QTX at 58 microsatellite markers across the genome. Based on the Map Manager QTX permutation test (1-cM intervals), the threshold for significance (P < 0.05) is an LRS score greater than 11.2. The CFU counts of all 95 mice based on their genotypes at the two suggestive loci on chromosomes 2 (B) and 12 (C), as well as a single significant locus on chromosome 17 (D), are shown.

FIG. 4.

Interval mapping at prl1 in F₂ and F₁ backcrossed mice. The results of the F₂ (A) and an F₁ backcross (C57BL/6J × CB6/F1J) (B) were subjected to interval mapping using QTL Cartographer across the first 45 cM of chromosome 17 in 1-cM intervals. The estimated peak LOD scores are located at 22 cM for the F₂ mice and at 24 and 30 cM for the F₁ backcross.

FIG. 5.

prl1 confers resistance to Y. pestis in a susceptible C57BL/6J background. Eighth generation backcrossed mice were intercrossed, generating prl1^+/+ (8 mice) and prl1^+/− (6 mice) animals. Mice were infected with 2,000 CFU Y. pestis along with parental controls (3 each). At 48 h postinfection, mice were sacrificed, and total splenic CFU was determined. Horizontal lines represent the average. The dashed line indicates the limit of detection. Asterisks indicate significant differences between groups as determined by a Student's t test where P is <0.05.