Eosinophils and Bacteria, the Beginning of a Story

Abstract

Eosinophils are granulocytes primarily associated with TH2 responses to parasites or immune hyper-reactive states, such as asthma, allergies, or eosinophilic esophagitis. However, it does not make sense from an evolutionary standpoint to maintain a cell type that is only specific for parasitic infections and that otherwise is somehow harmful to the host. In recent years, there has been a shift in the perception of these cells. Eosinophils have recently been recognized as regulators of immune homeostasis and suppressors of over-reactive pro-inflammatory responses by secreting specific molecules that dampen the immune response. Their role during parasitic infections has been well investigated, and their versatility during immune responses to helminths includes antigen presentation as well as modulation of T cell responses. Although it is known that eosinophils can present antigens during viral infections, there are still many mechanistic aspects of the involvement of eosinophils during viral infections that remain to be elucidated. However, are eosinophils able to respond to bacterial infections? Recent literature indicates that Helicobacter pylori triggers TH2 responses mediated by eosinophils; this promotes anti-inflammatory responses that might be involved in the long-term persistent infection caused by this pathogen. Apparently and on the contrary, in the respiratory tract, eosinophils promote TH17 pro-inflammatory responses during Bordetella bronchiseptica infection, and they are, in fact, critical for early clearance of bacteria from the respiratory tract. However, eosinophils are also intertwined with microbiota, and up to now, it is not clear if microbiota regulates eosinophils or vice versa, or how this connection influences immune responses. In this review, we highlight the current knowledge of eosinophils as regulators of pro and anti-inflammatory responses in the context of both infection and naïve conditions. We propose questions and future directions that might open novel research avenues in the future.

BordetellaSpp. Utilize T3SS To Promote Secretion of IL1RA Leading to Long-Term Persistent Infections

“…It is known that eosinophils prevent auto-inflammatory disorders on the mucosal surfaces, such as inflammatory bowel syndrome in the guts 25,26 by secreting Interleukin-1 receptor antagonist (IL1RA) [27][28][29] , Program death ligand 1 (PD-L1) 17,22 , and Indoleamine 2,3-dioxygenase (IDO) 13,30 .…”

“…Their anti-inflammatory function is performed through the secretion three main molecules; PDL1 (program death ligand-1), IDO (Indoleamine 2,3-dioxygenase), and IL1RA (IL1 receptor antagonist) 13 . Eosinophils secrete one or more of these molecules to dampen proinflammatory responses and restore the immune homeostatic balance.…”

“…Eosinophils are known for their function during parasitic infections as well as for their implications during allergic and asthmatic reactions 13 . The fact that eosinophils can kill bacteria has long been known 14 however it has not gained significant attention until recently, when the role of eosinophil traps during infections, allergic reactions, and asthmatic processes was studied indepth 15,16 .…”

“…The fact that eosinophils can kill bacteria has long been known 14 however it has not gained significant attention until recently, when the role of eosinophil traps during infections, allergic reactions, and asthmatic processes was studied indepth 15,16 . Eosinophils are critical for immune homeostasis to keep the balance between Th1 and Th2 responses; in fact, they are pivotal to prevent mucosal autoinflammatory disorders 13 . Their anti-inflammatory function is performed through the secretion three main molecules; PDL1 (program death ligand-1), IDO (Indoleamine 2,3-dioxygenase), and IL1RA (IL1 receptor antagonist) 13 .…”

Bordetella spp. block eosinophil recruitment to suppress lung iBALT formation

“…27.509784 doi: bioRxiv preprint RB50ΔbtrS persists in the lungs for nearly as long as the infection caused by the wild-type bacteria 18 , suggesting a role for eosinophils during adaptive immune responses. This type of granulocytes is well known in the context of parasitic infections and allergic and asthmatic reactions 19 . However, the role of eosinophils during health and disease has been long questioned by many researchers [20][21][22] .…”