An interaction between aromatic rings drives the recognition of microplastics by macrophages

Microplastics, resulting from plastic waste degradation, are ubiquitous. They likely enter the human body via diet and inhalation, whereby they get engulfed by macrophages. However, it is unclear how macrophages recognize microplastics. Now, scientists in Japan have discovered that Tim4, a receptor that recognizes apoptotic cells, facilitates the interaction between microplastics and macrophages and may hamper an important cellular process called efferocytosis. Their findings shed light on the absorption, distribution, and potential toxicity of microplastics.

A large portion of the nearly 200 million tons of single-use plastic produced globally ends up discarded as waste. The eventual breakdown of this plastic into microplastics–0.1–1000 µm-sized particles–is a cause for concern. Humans are exposed to microplastics not only through food consumption but also via inhalation, and their occurrence in the bloodstream and organs has already been documented. It is unknown if chronic exposure to microplastics impact human health, but they are targeted for phagocytosis–the process of foreign particle engulfment–by macrophages. Apart from pathogens, macrophages recognize environmental particles like nanomaterials by recognizing patterns in specific parts of targets slated for phagocytosis. While considered bioinert and non-inflammatory, it is not known how macrophages recognize microplastics and how their own function is impacted following microplastic engulfment.

Now, a group of researchers at Ritsumeikan University and Shimane University have shed light on how polystyrene (PS) microplastics interact with macrophages. Their findings were published in the journal Science of the Total Environment . This paper was made available online on March 3, 2023, and will be published in Volume 875 of the journal on June 1, 2023.

The research group, led by Professor Masafumi Nakayama from Ritsumeikan University, discovered the role of a previously unknown aromatic-aromatic interface between PS microplastics and the T cell immunoglobulin mucin 4 (Tim4) receptor on macrophages. Their previous research revealed that Tim4 recognizes carbon nanotubes (particles that cannot be digested following phagocytosis but provoke inflammation). "We were keen to see if Tim4 bound to PS which is composed of aromatic styrene units, and if this interaction perturbed an immune response," says Prof Nakayama when quizzed on his motivation for the study.

The group leveraged molecular techniques and mouse and human cell lines to study PS microplastic recognition. They measured efferocytosis (the macrophage engulfment of apoptotic cells by macrophages) and monitored the immune response by profiling reactive oxygen species (ROS), nitric oxide (NO), interleukin-1 beta (IL-1β), and tumor necrosis factor alpha (TNF-α) production following exposure to PS.

The group found that macrophages generated as Tim4-deletion mutants were not able to engulf PS microplastics or multi-walled carbon nanotubes (MWCNTs), thereby confirming the role of Tim4 in PS recognition. Elaborating on the main findings, Prof Nakayama says, "While MWCNT engulfment did induce the production of an immune response, PS microplastics used in this study did not induce NO, ROS, or TNF-α release. This shows that PS microplastics are not inflammatory and that macrophages can discriminate between particulates too."

The findings revealed that the binding site on Tim4 contained an aromatic cluster on its extracellular loop that bound to PS, and furthermore, that PS microplastics competitively blocked the process of efferocytosis. This is significant as it highlights that PS microplastics do not directly promote acute inflammation; however, exposure to large quantities of PS microplastics may cause chronic inflammation, leading to autoimmune diseases.

The group is excited by their discovery of the novel interface between microplastics and a biological system. Regarding future research avenues, Prof Nakayama concludes, "We have laid the groundwork to provide a better understanding of the absorption, distribution, excretion, and toxicity of bioavailable PS microplastics."

Title of original paper: Tim4, a macrophage receptor for apoptotic cells, binds polystyrene microplastics via aromatic-aromatic interactions
Journal: Science of the Total Environment
DOI: 10.1016/j.scitotenv.2023.162586

About Professor Masafumi Nakayama from Ritsumeikan University, Japan

Masafumi Nakayama graduated from Tohoku University and then earned his Doctor of Medicine degree from the Graduate School of Medicine at Juntendo University. He is currently a Professor in the Faculty of Pharmaceutical Sciences at Ritsumeikan University. His deep interest in the relationship between the immune system and disease drew him to a career in research, and he currently specializes in the analysis of biological defense mechanisms against pathogens. He has published 44 research articles and five books.

Funding information

The study was supported by Japan Society for the Promotion of Sciences grants JP22H03340 to Masafumi Nakayama, JP22H03969 to Nobuyuki Morimoto, and the Naito Foundation to Masafumi Nakayama