Adaptive immunity in Mus musculus influences the acquisition and abundance of Borrelia burgdorferi in Ixodes scapularis ticks

Abstract

The Lyme disease spirochete Borrelia burgdorferi cycles between immature black-legged ticks (Ixodes scapularis) and vertebrate reservoir hosts, such as rodents. Larval ticks acquire spirochetes from infected hosts, and the resultant nymphs transmit the spirochetes to naïve hosts. This study investigated the impact of immunocompetence and host tissue spirochete load on host-to-tick transmission (HTT) of B. burgdorferi and the spirochete load inside immature I. scapularis ticks. Wild-type (WT) C57BL/6J mice and mice with severe combined immunodeficiency (SCID) were experimentally infected with B. burgdorferi. To measure HTT, WT and SCID mice were repeatedly infested with I. scapularis larvae, and ticks were sacrificed at three different developmental stages: engorged larvae, 1-month-old, and 12-month-old nymphs. The spirochete loads in immature ticks and mouse tissues were estimated using qPCR. In WT mice, HTT decreased from 90% to 65% over the course of the infection, whereas in the SCID mice, HTT was always 100%. Larvae that fed on SCID mice acquired a much larger dose of spirochetes compared to larvae that fed on WT mice. This difference in spirochete load persisted over tick development where nymphs that fed as larvae on SCID mice had significantly higher spirochete loads compared to their WT counterparts. HTT and the tick spirochete loads were strongly correlated with the mouse tissue spirochete loads. Our study shows that the host immune system (e.g., the presence of antibodies) influences HTT of B. burgdorferi and the spirochete load in immature I. scapularis ticks.IMPORTANCEThe tick-borne spirochete Borrelia burgdorferi causes Lyme disease in humans. This pathogen is maintained in nature by cycles involving black-legged ticks and wildlife hosts. The present study investigated the host factors that influence the transmission of B. burgdorferi from infected hosts to feeding ticks. We infected immunocompetent mice and immunocompromised mice (that cannot develop antibodies) with B. burgdorferi and repeatedly infested these mice with ticks. We determined the percentage of infected ticks and their spirochete loads. This percentage was 100% for immunocompromised mice but decreased from 90% to 65% over time (8 weeks) for immunocompetent mice. The tick spirochete load was much higher in ticks fed on immunocompromised mice compared to ticks fed on immunocompetent mice. In summary, the host immune system influences the transmission of B. burgdorferi from infected hosts to ticks and the spirochete loads in those ticks, which, in turn, determines the risk of Lyme disease for people.

Keywords: Borrelia burgdorferi; C57BL/6J mouse; Ixodes scapularis; Lyme borreliosis; SCID mouse; host-to-tick transmission; tick spirochete load; tick-borne pathogen; vector-borne pathogen.

Fig 1

The infection prevalence of B. burgdorferi strain I in immature I. scapularis ticks in experiment I. The three tick stages on the X-axis are the engorged larvae, 1-month-old nymphs, and 12-month-old nymphs. The ticks that fed on the WT and SCID mice are shown in green and purple, respectively. Larval infestations 1, 2, and 3 are shown in the left, middle, and right panels, respectively. Larval infestations 1, 2, and 3 took place on days 28, 56, and 84 post-infection, respectively. The error bars show the means and their 95% confidence intervals.

Fig 2

The spirochete load of the immature I. scapularis ticks in experiment I. The spirochete load has units of 23S rRNA gene copies per tick. The three tick stages on the X-axis are the engorged larvae, 1-month-old nymphs, and 12-month-old nymphs. The ticks that fed on the WT and SCID mice are shown in green and purple, respectively. Larval infestations 1, 2, and 3 are shown in the left, middle, and right panels, respectively. Larval infestations 1, 2, and 3 took place on days 28, 56, and 84 post-infection, respectively. The black error bars show the means and their 95% confidence intervals. The data points show the spirochete loads of the individual ticks.

Fig 3

Relationship between mouse ear biopsy spirochete load and (A) the larval infection prevalence (LIP) and (B) the log10-transformed larval spirochete load (LSL) for experiment I. The mouse ear biopsy spirochete load is the number of 23S rRNA copies per million mouse Beta actin copies and is log10-transformed. The LIP is the percentage of fed larvae that acquired B. burgdorferi for each unique combination of mouse and infestation. The LSL is the mean number of 23S rRNA gene copies per infected larva for each unique combination of mouse and infestation. WT and SCID mice are shown by circles and diamonds, respectively. Larval infestations 1, 2, and 3, which occurred on days 28, 56, and 84 post-infection, are shown in blue, pink, and green, respectively.

Fig 4

Pairwise correlation matrix of 8 infection variables across the 32 B. burgdorferi-infected mice in experiment I. The variables included (i) number of engorged larvae recovered from each mouse during the larval infestation, (ii) mouse ear biopsy spirochete load, (iii) spirochete load in mouse heart, (iv) LIP, (v) LSL, (vi) NIP, (vii) NSL in 1-month-old nymphs, and (viii) NSL in 12-month-old nymphs. Pairwise correlations marked with an “X” are not statistically significant (P > 0.05). The color and the size of the circles indicate the direction and the magnitude of the Pearson correlation coefficient (i.e., blue and red are positive and negative correlations, respectively).

Fig 5

The tick spirochete load is shown for 11 different strains of B. burgdorferi in engorged larvae (red bars) and unfed 1-month-old nymphs (gray bars). Larval I. scapularis ticks were fed on C3H/HeJ mice infected with one of the 11 strains of B. burgdorferi. Engorged larvae were frozen following drop-off or allowed to molt into nymphs and frozen 4 weeks after ecdysis. The tick spirochete load was estimated using the 23S rRNA qPCR assay. Tick spirochete load therefore has units of the number of 23S rRNA copies per tick. The bars represent the 95% confidence intervals of the mean.