Francisella tularensis novicida infection competence differs in cell lines derived from United States populations of Dermacentor andersoni and Ixodes scapularis

In the United States, Dermacentor spp. are common vectors of Francisella tularensis subspecies (ssp.), while Ixodes scapularis is not, though the geographic distribution and host range of pathogen and tick overlap. To examine if differences in infection competence at the cellular level underpin these ecological differences, we evaluated the competence of D. andersoni (DAE100) and I. scapularis (ISE6) cell lines to support F. tularensis ssp. novicida (F. novicida) infection. Importantly, D. andersoni is a vector for both F. tularensis spp. tularensis, and F. novicida. We hypothesized F. novicida infection would be more productive in D. andersoni than in I. scapularis cells. Specifically, we determined if there are differences in F. novicida i) invasion, ii) replication, or iii) tick cell viability between DAE100 and ISE6 cells. We further examined the influence of temperature on infection kinetics. Both cell lines were permissive to F. novicida infection; however, there were significantly higher bacterial levels and mortality in DAE100 compared to ISE6 cells. Infection at environmental temperatures prolonged the time bacteria were maintained at high levels and reduced tick cell mortality in both cell lines. Identifying cellular determinants of vector competence is essential in understanding tick-borne disease ecology and designing effective intervention strategies.