On April 12, 2020, the World Health Organization (WHO) published Environmental Health Criteria 244:  Principles and methods to assess the risk of immunotoxicity associated with exposure to nanomaterials.  WHO states that the document presents an overview of the current knowledge and evidence on principles and basic mechanisms of immunotoxicity caused by engineered nanomaterials (ENM).  The document provides guidance on principles and methods for hazard and risk assessment of different ENMs and groups of ENMs on the immunological system in the body; describes the key cell types and elements and the functioning of the human immunological system; and provides information on the effects of various ENMs on these cells and elements of the immune system.  It includes case studies on carbon nanotubes and silver nanoparticles to illustrate how the information derived from the application of currently available methods can be structured to assess the hazards in terms of their immunotoxic properties.  The document includes the following recommendations:

• Include nanoimmunotoxicity risk assessment in general nanotoxicity risk assessment;
• Define the minimal requirements for characterization of test materials before, during, and after the experiments, and control the dispersion of ENMs for both in vivo and in vitro testing;
• Define reference materials for further testing, standardization, and validation of test systems;
• Advance testing for effects of nanomaterials by implementing developments in the fields of molecular biology, systems biology, bioinformatics, high-throughput screening, and in silico modeling;
• Validate test methods for ENMs by ensuring that the test material itself does not interfere with the assay, and establish validated protocols for endotoxin testing of ENMs;
• Advance approaches to relevant exposure regimens, both in vivo and in vitro, and define the most appropriate dose metrics (particle mass, number, surface area);
• Identify positive controls for various bioassays;
• Explore absorption, distribution, metabolism, and excretion of ENMs on a case-by-case basis, and investigate how interactions with the immune system affect the toxicokinetics of ENMs;
• Develop more advanced nanomaterial-relevant models to study immunotoxicity, including more advanced in vitro model systems to study the immunotoxicity of ENMs, taking into account the interplay between immune cell populations and other cells (for example, lung epithelial cells), as well as the role of the biocorona on the surface of ENMs;
• Unravel mechanisms of ENM immunotoxicity and develop adverse outcome pathways, and define patterns of responses applying to different groups of ENMs to facilitate risk assessment of ENMs;
• Develop an intelligent, mechanism-based, tiered testing strategy for ENMs taking into account the four fundamental steps of traditional risk assessment (hazard identification, hazard characterization, exposure assessment, and risk characterization);
• Identify susceptible populations at greatest risk for potential adverse effects of nanomaterials on immune-mediated disease (taking account of such criteria as sex, age, ethnicity, pre-existing condition);
• Develop approaches to immunotoxicology of complex mixtures of ENMs and other compounds;
• Establish acceptable uncertainty associated with exposure to nanomaterials; and
• Adapt Organization for Economic Cooperation and Development (OECD) test guidelines related to immune toxicity to include the assessment of nanomaterials.