Roosting ecology and variation in adaptive and innate immune system function in the Brazilian free-tailed bat (Tadarida brasiliensis)

Abstract

Bats have recently been implicated as reservoirs of important emerging diseases. However, few studies have examined immune responses in bats, and even fewer have evaluated these responses in an ecological context. We examined aspects of both innate and adaptive immune response in adult female Brazilian free-tailed bats (Tadarida brasiliensis) at four maternity roosts (two natural caves and two human-made bridges) in south-central Texas. Immune measurements included in vitro bactericidal ability of whole blood and in vivo T cell mediated response to mitogenic challenge. Bactericidal activity in T. brasiliensis varied with roosting ecology, but appears to be sensitive to colony-level effects. Blood from females living at one cave had significantly lower bactericidal ability than blood from females at three other sites. T cell mediated response in this species was associated with variation in roost ecology, with females from two caves having greater responses than females from two bridges. T cell mediated response and bactericidal activity were negatively correlated with one another within individuals that were tested for both. Variation in immunological response of T. brasiliensis is important for understanding the influence of the environment on the frequency and distribution of immunologically competent individuals and for understanding disease-host dynamics in this and other colonial species.

Fig. 1

a BKA Index (bactericidal ability of blood; mean ± SE) at the four colonies sampled, blood from bats at Davis Cave had significantly lower bactericidal ability than blood from bats at the other colonies (P < 0.05). Caves versus bridges (P = 0.06). Sample sizes for Frio and Davis Caves and Seco Creek and East Elm Creek Bridges are n = 30, 34, 11, 14, respectively. b PHA Index (response to PHA injection at 12 h; mean ± SE) at the four colonies sampled. Letters denote statistical differences between colonies, estimated using post-hoc analysis. Bats sampled at two cave colonies had significantly greater response to PHA challenge than those at the two bridges (colony, P = 0.02; roost type, P = 0.001). Sample sizes for Frio and Davis Caves and Seco Creek and East Elm Creek Bridges are n = 50, 76, 24, 12, respectively

Fig. 2

BKA index (bactericidal ability of blood) as a function of PHA index (response to PHA injection at 12 h) in individual adult female bats tested for both. There was a negative correlation between BKA and PHA indices (Pearson’s ρ = −0.190, P = 0.05)

Fig. 3

Mean PHA index (response to PHA injection at 12 h) as a function of estimated colony size. Colonies estimates in order from largest to smallest are Frio Cave, Davis Cave, Seco Creek Bridge and East Elm Creek Bridge. The positive correlation between colony size and mean PHA Index was significant (P < 0.0001, F = 19.79(3, 161), R ² = 0.11). Sample sizes for Frio and Davis Caves and Seco Creek and East Elm Creek Bridges are n = 50, 76, 24, 12, respectively