Mastitis Increases Mammary mRNA Abundance of β-Defensin 5, Toll-Like-Receptor 2 (TLR2), and TLR4 but Not TLR9 in Cattle

TLDR

Primary defense against pathogens relies on pathogen‑recognition receptors that trigger early innate immune effector release. The study aimed to characterize bovine TLR9 and map its chromosomal location to assess its role in mastitis defense. Real‑time PCR assays were developed to quantify mRNA levels of TLR9, TLR2, TLR4, and β‑defensin 5 in healthy and infected mammary glands. Mastitis caused 4‑ to 13‑fold increases in TLR2, TLR4, and β‑defensin 5 mRNA, but not TLR9, with subclinical infections inducing smaller rises; TLR2 and TLR4 were coordinately regulated and localized to mammary epithelial cells, underscoring the innate immune system’s role in countering udder infection.

Abstract

ABSTRACT Coordination of the primary defense mechanisms against pathogens relies on the appropriate expression of pathogen recognition receptors (PRRs) triggering the early release of effector molecules of the innate immune system. To analyze the impact of this system on the counteraction of infections of the mammary gland (mastitis), we characterized the bovine gene encoding the key PRR Toll-like receptor 9 (TLR9) and mapped its precise position on chromosome BTA22. The sequence information was used to establish real-time PCR quantification assays to measure the mRNA abundances of TLR9, TLR2, and TLR4 together with those of β-defensin 5 (BNBD5), an early bactericidal effector molecule of the innate system, in healthy and infected mammary glands. Mastitis strongly increased (4- to 13-fold) the mRNA abundances of all of these genes except TLR9. Slight subclinical infections already caused a substantial increase in the copy numbers, though they did so the least for TLR9. Induction was not systemic, since mRNA abundance was low in uninfected control quarters of the udder but high in the severely infected quarters of the same animal. The number of TLR2 copies correlated well with those of TLR4, indicating coordinated regulation of these two PRRs during infection of the udder. Their coordinated regulation explains our unexpected observation that pure Staphylococcus aureus infections caused a strong increase also in TLR4 mRNA abundance. In situ hybridizations revealed that BNBD5 is expressed predominantly in the mammary epithelial cells (MEC) of the infected gland. Our data therefore suggest a significant contribution of the innate immune system to counteract mastitis and attribute a prominent effector function to the MEC.