Abstract
Toll-like receptors (TLRs) are known to detect a defined spectrum of microbial structures. However, the knowledge about the specificity of teleost Tlr factors for distinct pathogens is limited so far. We measured baseline expression profiles of 18 tlr genes and associated signaling factors in four immune-relevant tissues of rainbow trout Oncorhynchus mykiss. Intraperitoneal injection of a lethal dose of Aeromonas salmonicida subsp. salmonicida induced highly increased levels of cytokine mRNAs during a 72-hour postinfection (hpi) period. In contrast, only the fish-specific tlr22a2 and the downstream factor irak1 featured clearly increased transcript levels, while the mRNA concentrations of many other tlr genes decreased. Flow cytometry quantified cell trafficking after infection indicating a dramatic influx of myeloid cells into the peritoneum and a belated low level immigration of lymphoid cells. T and B lymphocytes were differentiated with RT-qPCR revealing that B lymphocytes emigrated from and T lymphocytes immigrated into head kidney. In conclusion, no specific TLR can be singled out as a dominant receptor for A. salmonicida. The recruitment of cellular factors of innate immunity rather than induced expression of pathogen receptors is hence of key importance for mounting a first immune defense against invading A. salmonicida.
Highlights
The vertebrate immune system consists of a conserved innate system complemented by a highly specialized immune system
T and B lymphocytes were differentiated with RT-qPCR revealing that B lymphocytes emigrated from and T lymphocytes immigrated into head kidney
Regarding the tissue-specific expression, we found that all 13 tlr genes were significantly expressed in spleen, head kidney, liver, and thymus (Figure 2(a))
Summary
The vertebrate immune system consists of a conserved innate system complemented by a highly specialized (adaptive) immune system. Both branches of immunity communicate and collaborate in a bipartisan way to ensure the effective destruction of potentially harmful microbes [1]. More than 20 TLRs clustered in six subfamilies have been identified in more than a dozen of fish species [5,6,7] (Figure 1).
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