Figure 5.

T cells do not influence bluetongue virus (BTV) infection rate of Culicoides sonorensis feeding on viraemic sheep. (A, B) Uninfected, colonised C. sonorensis were fed on BTV-infected sheep (n = 28) at peak viraemia to determine whether absence of individual T-cell subsets (CD4⁺, CD8⁺, and WC1⁺ γδ) influenced onwards transmission of BTV to a susceptible Culicoides vector. (A) Log₁₀ BTV genome copies in individual engorged female C. sonorensis midges incubated for 8 days (triangles) at 25°C following blood-feeding on CD4⁺ (purple, n = 345), CD8⁺ (green, n = 345), or WC1⁺ γδ (blue, n = 338) T cell- or mock-depleted (red, n = 357) sheep at peak viraemia during infection with BTV-4 MOR2009/07. Baseline viral uptake (median, black line; range, grey shading) was determined by quantifying the log₁₀ BTV genome copies per midge in identically fed C. sonorensis (n = 88, 80, 88, and 104, respectively) processed immediately (day 0) following feeding on the sheep. (B) Variation in infection rate (proportion of BTV-positive midges, %) of C. sonorensis fed on individual BTV-4 infected sheep (dots; median, black line) following CD4⁺ (n = 7), CD8⁺ (n = 7), or WC1⁺ γδ (n = 7) T cell or mock (n = 7) depletion. No statistically significant differences were identified in infection rate between depletion groups using median viral uptake of matched day 0 midges as cut-off for viral replication, as determined using binomial family generalised linear mixed model with logit link function. (C, D) Linear regression of the relationship between viral load in CD4⁺, CD8⁺, or WC1⁺ γδ T cell- and mock-depleted sheep on the day of midge feeding (peak viraemia) and viral load in the (C) day 0 and (D) day 8 incubated C. sonorensis. Log₁₀ BTV genome copies detected in individual insects (dots) and the mean of all insects fed on individual sheep (circles) are shown alongside the best-fit linear regression line. (E) Logistic regression of the relationship between infection rate (proportion of BTV-positive midges, %) in C. sonorensis fed on individual CD4⁺ (n = 7), CD8⁺ (n = 7), or WC1⁺ γδ (n = 7) T cell- or mock-depleted (n = 7) sheep (circles) and the viral load (log₁₀ BTV genome copies per ml blood) in individual sheep at the time of Culicoides feeding. The fitted black line demonstrates a positive, but not significant (p = 0.200), relationship between Culicoides infection rate and infectious blood meal.