Defects in host immune function in tree frogs with chronic chytridiomycosis

Abstract

The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has caused mass mortality leading to population declines and extinctions in many frog species worldwide. The lack of host resistance may be due to fungal immunosuppressive effects that have been observed when Bd is incubated with cultured lymphocytes, but whether in vivo host immunosuppression occurs is unknown. We used a broad range of hematologic and protein electrophoresis biomarkers, along with various functional tests, to assess immune competence in common green (Litoria caerulea) and white-lipped (L. infrafrenata) tree frogs experimentally infected with Bd. Compared with uninfected frogs, Bd infection in L. caerulea caused a reduction in immunoglobulin and splenic lymphocyte responses to antigenic stimulation with sheep red blood cells, along with decreased white blood cell and serum protein concentrations, indicating possible impaired immune response capability of Bd-infected frogs. This is the first in vivo study suggesting that infection with Bd causes multiple defects in systemic host immune function, and this may contribute to disease development in susceptible host species. Although L. infrafrenata failed to maintain Bd infection after exposure, white blood cell and serum globulin concentrations were lower in recovered frogs compared with unexposed frogs, but antigen-specific serum and splenic antibody, and splenic cellular, responses were similar in both recovered and unexposed frogs. This may indicate potential systemic costs associated with infection clearance and/or redirection of host resources towards more effective mechanisms to overcome infection. No clear mechanism for resistance was identified in L. infrafrenata, suggesting that localized and/or innate immune defense mechanisms may be important factors involved in disease resistance in this species.

Figure 1. White blood cell counts of Litoria caerulea.

Total white blood cell (WBC) (Fig. 1A) and lymphocyte (Fig. 1B) counts (x10⁹ l⁻¹) pre- and post-immune stimulation in healthy uninfected (n = 10) and Batrachochytrium dendrobatidis-infected (n = 10) Litoria caerulea. Bars are mean ± SEM. *P<0.050 within each group.

Figure 2. Serum globulin fractions of Litoria caerulea.

Serum total globulins (Fig. 2A), α-1 globulin (Fig. 2B) and γ globulin (Fig. 2C) concentrations (g l⁻¹) pre- and post-immune stimulation in healthy uninfected (n = 10) and Batrachochytrium dendrobatidis-infected (n = 10) Litoria caerulea. Bars are mean ± SEM. *P<0.050 within each group.

Figure 3. White blood cell counts of Litoria infrafrenata.

Total white blood cell (WBC) (Fig. 3A) and neutrophil (Fig. 3B) counts (x10⁹ l⁻¹) pre- and post-immune stimulation in healthy unexposed (n = 10) and Batrachochytrium dendrobatidis-exposed but uninfected (n = 9) Litoria infrafrenata. Bars are mean ± SEM. *P<0.050 within each group.

Figure 4. Serum α-1 globulins of Litoria infrafrenata.

Serum α-1 globulin concentration (g l⁻¹) pre- and post-immune stimulation in healthy unexposed (n = 10) and Batrachochytrium dendrobatidis-exposed but uninfected (n = 9) Litoria infrafrenata. Bars are mean ± SEM. *P<0.050 within each group.