Abstract
Granulocytes, the most abundant types of leukocytes, are the first line of defense against pathogen invasion. However, the plasticity and diversity of granulocytes have been increasingly revealed, especially with regard to their versatile functions in orchestrating adaptive immune responses. A substantial body of recent evidence demonstrates that granulocytes can acquire the function as antigen-presenting cells under pathological or inflammatory conditions. In addition, they can acquire surface expression of MHC class II and costimulatory molecules as well as T cell stimulatory behavior when cultured with selected cytokines. The classic view of granulocytes as terminally differentiated, short-lived phagocytes is therefore changing to phenotypically and functionally heterogeneous cells that are engaged in cross-talk with other leukocyte populations and provide an additional link between innate and adaptive immunity. In this brief review, we summarize the current knowledge on the antigen-presenting capacity of granulocyte subsets (neutrophils, eosinophils, and basophils). Underlying mechanisms, relevant physiological significance and potential controversies are also discussed.
Highlights
As key components in the innate immune system, granulocytes have generally been considered as rapid responders in the first line of defense against pathogens
Discrepancies exist with respect to how the properties of granulocytes change under different circumstances and the models used. To this end, resting neutrophils show no or very low expression of MHC-II and have been shown to be unable to stimulate proliferation of naïve CD4+ T cells in a mixed lymphocyte reaction [6]. This suggests that neutrophils are unable to, or at least inefficient at priming naïve T cell responses, which is in contrast to classical antigen-presenting cells (APCs)
We recently showed that fresh human neutrophils can present cognate protein antigens to autologous antigen-specific CD4+ T cells [6]
Summary
As key components in the innate immune system, granulocytes have generally been considered as rapid responders in the first line of defense against pathogens. The role of granulocytes has long been considered restricted to the initial phase of the defense. A substantial body of evidence has indicated that there is a functional heterogeneity and plasticity among granulocytes, with most emphasis on their versatile abilities in shaping adaptive immune responses [1, 2]. To this end, T cell responses orchestrated by granulocytes via antigen presentation have been described and received considerable attention. We will discuss the potential underlying mechanisms and their physiological significance
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