Salivary cholinesterases of blood feeding arthropod vectors: A likely modulator of host immune responses

Authors: Temeyer, Kevin; Schlechte, Kristie; Saelao, Perot

Submitted to: American Chemical Society National Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 6/26/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Blood feeding arthropod parasites (such as ticks, mosquitoes and sand flies) are important agents transmitting vector-borne diseases. Vector-borne diseases constitute about 17% of all human infectious diseases, worldwide, in addition to animal diseases. The "One Health" concept, summarizes that pathogens causing animal diseases are an important source for new diseases transmissible to humans. Blood feeding arthropod vectors bypass the protective skin barrier to pathogen entry, facilitating pathogen transmission and development of infection. Although much is known with regard to host-protective immune responses and their regulation, much remains to be elucidated due to the molecular and cellular complexity of the immune system. In addition to providing a route of pathogen entry, arthropod vector saliva is known to contain components that facilitate pathogen transmission and development within host organisms, a phenomenon known as saliva-assisted transmission, or SAT. USDA scientists recently proposed that cholinesterase enzymes in vector saliva may modulatw host immune responses, likely acting to protect parasitic success and facilitating pathogen establishment in the host. The presence of cholinesterase enzymes was documented in saliva of several blood feeding arthropod vectors, but absent in several non-vector blood feeding arthropods. The salivary cholinesterases could alter host immune responses by hydrolysing acetylcholine at the arthropod bite site, thereby altering activation and expression of host immune cell responses. Acetylcholine is present in most tissues, where it helps to maintain health of cells and tissues. An initial step in immune cellactivation is the production and secretion of acetylcholine, which acts as a regulator of cellular activity and may initiate processes to recruit other immune cells. Dendritic and macrophage cells are believed to functional as immune sentinels, acting to alert and direct immune responses. We utilized RNA sequencing to examine potential changes in transcript expression in murine dendritic cells derived from bone marrow, resulting from immune activation by lipopolysaccharide (LPS) and lipoteichoic acid (LTA) in the presence of active or inactive recombinant tick salivary cholinesterase. It was found that although the dendritic cells responded to immune activation, there was no difference in transcript expression between dendritic cultures simultaneously exposed to either active, or inactive tick cholinesterase enzymes. Further examination revealed that the dendritic cells failed to express choline acetytransferase (ChAT), an enzyme needed for acetylcholine synthesis. Results of further experiments are discussed that utilizing macrophage cells known to increase ChAT expression in response to LPS stimulation.

Technical Abstract: Blood feeding ectoparasitic arthropods are important agents for transmission of pathogens causing vector-borne diseases. Vector-borne diseases constitute about 17% of all human infectious diseases, worldwide, in addition to diseases in animals, Under the "One Health" concept, pathogens causing disease in animals serve as a source for new zoonotic diseases transmissible to humans, or human diseases may also become transmissible to animals. In addition to pathogen transmission by direct contact, blood feeding arthropod vectors provide a means of bypassing the skin barrier to pathogen entry and development of infection. The known mechanisms of host-protective immune responses are rapidly increasing; however, much remains to be elucidated due to the molecular and cellular complexity of the immune system. In addition to providing a route of pathogen entry, the saliva of arthropod vectors is known to contain various components capable of facilitating successful transmission and development of pathogens within host organisms, a phenomenon known as saliva-assisted transmission, or SAT. We recently hypothesized that cholinesterase enzymes in vector saliva are likely modulators of host immune responses, acting to protect parasitic success and potentially facilitating pathogen transmission and establishment in the host. We subsequently documented the presence of cholinesterase enzymes in saliva of several blood feeding arthropod vectors, absent in several non-vector blood feeding arthropods. The expected mechanism by which the salivary cholinesterases could effectively modulate host immune responses would be by hydrolysis of host tissue acetylcholine, thereby altering activation of nicotinic and muscarinic acetylcholine receptors in host immune cells. Acetylcholine is present at various levels in most tissues, helping to maintain cellular and tissue homeostasis. One of the initial steps in activation of immune cells is the production and secretion of acetylcholine, which could act as an autocrine or paracrine regulator of cellular activity and interaction and recruitment of other immune cells. Dendritic and macrophage cells are believed to functional as immune sentinels, acting to alert and direct immune responses by components of the innate and acquired immune systems. We utilized RNA sequencing to examine potential changes in transcript expression in murine dendritic cells derived from bone marrow, resulting from immune activation by lipopolysaccharide (LPS) and lipoteichoic acid (LTA) in the presence of enzymatically-active or -inactive recombinant tick salivary cholinesterase. Although the dendritic cells responded to immune activation, there was no difference in transcript expression between dendritic cultures simultaneously exposed to either enzymatically-active, or -inactive tick cholinesterase. Further examination revealed that the dendritic cells failed to express choline acetytransferase (ChAT), the enzyme responsible for acetylcholine synthesis. Similar experiments are underway using the murine macrophage cell line RAW264.7, which is known to respond to LPS stimulation by increased ChAT expression.