Characteristics of Borrelia hermsii infection in human hematopoietic stem cell-engrafted mice mirror those of human relapsing fever

Abstract

Rodents are natural reservoirs for a variety of species of Borrelia that cause relapsing fever (RF) in humans. The murine model of this disease recapitulates many of the clinical manifestations of the human disease and has revealed that T cell-independent antibody responses are required to resolve the bacteremic episodes. However, it is not clear whether such protective humoral responses are mounted in humans. We examined Borrelia hermsii infection in human hematopoietic stem cell-engrafted nonobese diabetic/SCID/IL-2Rγ(null) mice: "human immune system mice" (HISmice). Infection of these mice, which are severely deficient in lymphoid and myeloid compartments, with B. hermsii resulted in persistent bacteremia. In contrast, this infection in HISmice resulted in recurrent episodes of bacteremia, the hallmark of RF. The resolution of the primary episode of bacteremia was concurrent with the generation of B. hermsii-specific human IgM. Remarkably, HISmice generated antibody responses to the B. hermsii outer-membrane protein Factor H binding protein A. Sera from humans infected by B. hermsii have a similar reactivity, and studies in mice have shown that this response is generated by the B1b cell subset. HISmice contain several B-cell subsets, including those with the phenotype CD20(+)CD27(+)CD43(+)CD70(-), a proposed human equivalent of mouse B1 cells. Reduction of B cells by administration of anti-human CD20 antibody resulted in diminished anti-B. hermsii responses and persistent bacteremia in HISmice. These data indicate that analysis of B. hermsii infection in HISmice will serve as a model in which to study the cellular and molecular mechanisms involved in controlling human RF.

Fig. 1.

HISmice can resolve severe bacteremic episodes and mount B. hermsii-specific IgM responses. (A) WT (C57BL/6), NSG, or human hematopoietic stem cell-engrafted NSG (NSG+HSC) mice were infected i.v. with 5 × 10⁴ B. hermsii strain DAH-p1, and bacteremia was monitored by dark-field microscopy. Each plot represents data from an individual mouse. Representative data from three mice from a total of seven are shown (B) B. hermsii-specific IgM generated during an i.v. infection of WT (C57BL6) or HISmice (NSG+HSC) was measured by ELISA. Mean ± SD values are shown. (C) FhbA-specific IgM in preimmune (day 0 postinfection) or immune (1–3 wk postinfection) mice following i.v. infection was measured by ELISA. Data points represent individual animals, and horizontal bars represent means of each group. Statistical significance of genotype was measured by Student t test (***P < 0.001 and *P < 0.05).

Fig. 2.

Efficient resolution of moderate bacteremic episodes in HISmice. (A) WT (C57BL/6), NSG, or human hematopoietic stem cell-engrafted NSG (NSG+HSC) mice were infected intraperitoneally with 5 × 10⁴ B. hermsii strain DAH-p19, and bacteremia was monitored by dark-field microscopy. Each plot represents data from an individual mouse. Representative data from three mice from a total of six are shown. (B) FhbA-specific IgM in preimmune (day 0 postinfection) or immune (1–3 wk postinfection) mice following i.p. infection was measured by ELISA. Data points represent individual animals, and horizontal bars represent means of each group. Statistical significance of genotype was measured by Student t test (*P < 0.05 and **P < 0.01).

Fig. 3.

Analysis of B cells in HISmice. (A) Peritoneal cavity cells, (B) spleen cells, and (C) blood were stained with antibodies specific for human CD19, CD20, CD27, CD43, CD70, and CD69, and analyzed by flow cytometry. All B cells were first identified by CD19 positivity and were further resolved (indicated by arrows) as naive (CD20⁺CD27⁻), memory B cells (CD20⁺CD27⁺CD43⁻CD70⁻), and B1 cells (CD20⁺CD27⁺CD43⁺CD70⁻). As described previously (35), a majority of these B1 cells expressed CD5. The frequency values of the indicated B-cell populations are shown within the plots. The data were generated by analyzing a minimum of 20,000 cells and are representative of three to five mice. Five percent contour plots are shown. Immunohistochemistry of (D) spleen and (E) mesenteric lymph nodes of a representative mouse.

Fig. 4.

Depletion of B cells results in impaired anti-B. hermsii–specific IgM responses and persistent bacteremia. (A) HISmice treated with (n = 3; rituximab) or without (n = 3; control) anti-human CD20 mAb and B-cell populations in peritoneal cavity, spleen, and blood were determined by flow cytometry using human CD19 antibodies. Representative FACS plot from one mouse is shown. Control or rituximab-treated mice were infected i.v. with B. hermsii DAH-p1 and (B) bacteremia, (C) anti-B. hermsii–specific IgM, and (D) anti-FhBA–specific IgM.