Flow Cytometry for the Immunotoxicologist

Gender difference and BMDL exploration of developmental immunotoxicity induced by early-life low-dose exposure to 4-nonylphenol in Wistar rats

BACKGROUND

Nonylphenol (NP) is a widespread environmental endocrine disruptor with potential developmental immunotoxicity. The present study aimed to investigate the gender-specific developmental immunotoxic effects of early-life exposure to low doses of 4-nonylphenol (4-NP) on Wistar rats and the corresponding thresholds.

METHODS

Pregnant rats (F0 generation) were exposed to low doses of 4-NP from gestational day 6 (GD6) to postnatal day 90 (PND90), and F1 offspring continued to be exposed until the maturation of the immune system on PND42. We assessed immune organ development, immune responses, lymphocyte subset composition, cytokine secretion, and the Th17/Treg cell balance as endpoints for developmental immunotoxicity. Benchmark Dose analysis was conducted to explore the thresholds.

RESULTS

Early-life exposure to 4-NP led to significant gender-specific differences in the immune response. Female pups exhibited greater sensitivity to 4-NP, with reduced thymus and spleen weights, suppressed humoral immune function, decreased natural killer (NK) cell activity, and an imbalance in the Th17/Treg cell ratio. Male pups showed inhibition of NK cell activity but no significant changes in humoral immune function. Levels of phosphorylated STAT3, STAT5, and JAK3 proteins increased in the spleens of exposed pups of both gender. The lowest benchmark dose lower limit (BMDL) value of developmental immunotoxicity was lower in female rats (based on the thymus weight) than in male rats (based on the NK cell activity).

CONCLUSION

Early-life exposure to 4-NP has been shown to induce gender-specific developmental immunotoxicity in rats, with female pups exhibiting greater sensitivity. And developmental immunotoxicity may serve as a more sensitive indicator for the risk assessment of 4-NP. Th17/Treg balance may be interrupted through JNK/STAT pathway by 4-NP exposure, which needs to be further investigated.

[Evaluation of immunotoxicitiy in biomedical research and development]

Immunotoxicology aims at studying toxic effects of any substance on the immune system and its functions. In its various fields of application, this science is dependent on regulatory texts and guidelines. Studies are based on in vitro, ex vivo and in vivo techniques and are observational or functional allowing the identification of a toxic effect and its underlying mechanisms, respectively. Here, we review the various tests to perform in biomedical research and development, with a particular interest for the T-cell Dependent Antibody Response (TDAR) assay. We also briefly discuss the upcoming evolutions in this domain within a more ethically sound framework such as limiting the use of laboratory animals. These evolutions are represented by the development of relevant cell models.

In vivo assessment of respiratory burst inhibition by xenobiotic exposure using larval zebrafish

Currently, assessment of the potential immunotoxicity of a given agent involves a tiered approach for hazard identification and mechanistic studies, including observational studies, evaluation of immune function, and measurement of susceptibility to infectious and neoplastic diseases. These studies generally use costly low-throughput mammalian models. Zebrafish, however, offer an excellent alternative due to their rapid development, ease of maintenance, and homology to mammalian immune system function and development. Larval zebrafish also are a convenient model to study the innate immune system with no interference from the adaptive immune system. In this study, a respiratory burst assay (RBA) was utilized to measure reactive oxygen species (ROS) production after developmental xenobiotic exposure. Embryos were exposed to non-teratogenic doses of chemicals and at 96 h post-fertilization, the ability to produce ROS was measured. Using the RBA, 12 compounds with varying immune-suppressive properties were screened. Seven compounds neither suppressed nor enhanced the respiratory burst; five reproducibly suppressed global ROS production, but with varying potencies: benzo[a]pyrene, 17β-estradiol, lead acetate, methoxychlor, and phenanthrene. These five compounds have all previously been reported as immunosuppressive in mammalian innate immunity assays. To evaluate whether the suppression of ROS by these compounds was a result of decreased immune cell numbers, flow cytometry with transgenic zebrafish larvae was used to count the numbers of neutrophils and macrophages after chemical exposure. With this assay, benzo[a]pyrene was found to be the only chemical that induced a change in the number of immune cells by increasing macrophage but not neutrophil numbers. Taken together, this work demonstrates the utility of zebrafish larvae as a vertebrate model for identifying compounds that impact innate immune function at non-teratogenic levels and validates measuring ROS production and phagocyte numbers as metrics for monitoring how xenobiotic exposure alters the innate immune system.