The immune cell profile of the developing rat brain.

Area of Science:

• Neuroimmunology
• Developmental neuroscience
• Immunology

Background:

• Peripheral immune cells (PICs) play roles in the mature nervous system.
• Their presence and function in the developing brain remain largely unknown.
• Understanding PICs in brain development is crucial for studying neurological disorders.

Purpose of the Study:

• To characterize the peripheral immune cell profile in the developing rat brain.
• To map the location, abundance, and temporal changes of PICs during brain development.
• To investigate the impact of immune stimulation on neonatal brain PICs.

Main Methods:

• Spectral flow cytometry was used to analyze PICs in rat brain regions (cerebellum, hippocampus, hypothalamus).
• Samples were collected at embryonic day 20 (E20) and postnatal days 0 (PN0), 7 (PN7), and 16 (PN16).
• Poly I:C was administered to mimic viral infection and assess immune cell influx.

Main Results:

• PICs were most abundant at E20, with higher concentrations in the cerebellum compared to the hippocampus and hypothalamus.
• Monocytes were the most prevalent PICs, shifting from classical to non-classical phenotypes by PN7.
• T cells increased over time, transitioning from cytotoxic to T-helper dominance by PN7.
• Natural killer (NK) cells and mast cells showed transient increases; mast cells were localized to the hippocampus and hypothalamus.
• Poly I:C administration increased PIC influx, notably NK cells and non-classical monocytes (males only).

Conclusions:

• The peripheral immune cell profile in the developing rat brain undergoes a transition from a reactive to an adaptive and surveilling state during the second postnatal week.
• Specific PICs like mast cells may have region-specific roles in brain development.
• This study provides a foundational map of immune cell dynamics in the developing brain, aiding research into healthy and pathological development.