Neuroimmune Interactions
The neuroimmune coupling operates through three pathways, each with distinct timescales.
1 Cytokine Effects on Brain Function
Peripheral cytokines affect CNS function through two mechanisms modeled in Immune System Models: (1) transport across the blood–brain barrier (bbb transport), and (2) activation of vagal afferents that signal to the brainstem (Dantzer 2008). Once in the CNS, pro-inflammatory cytokines activate microglia (microglia), disrupt neurotransmitter synthesis (tryptophan and ido regulation), and alter HPA axis function (through direct effects on hypothalamic CRH neurons). The integrated model connects these pathways:
\[ \mathbf{C}_\text{CNS}(t) = P_\text{BBB}(t) \cdot \mathbf{C}_\text{plasma}(t) + \mathbf{T}_\text{active} + \mathbf{C}_\text{local}(\mu_1) \tag{1}\]
where \(\mathbf{C}_\text{local}(\mu_1)\) represents cytokines produced locally by activated microglia. neuroimmune cns is a quasi-steady-state approximation of the full transport dynamics (bbb transport), valid when CNS cytokine turnover is fast relative to BBB permeability changes. The CNS cytokine environment then modulates neurotransmitter dynamics, autonomic output, and HPA axis function—creating a pathway from peripheral immune activation to cognitive dysfunction, sleep disturbance, and autonomic symptoms.
2 HPA Axis and Immune Regulation
Cortisol from the HPA axis model (hpa axis) feeds back to the immune system as a major anti-inflammatory signal:
\[ \sigma_{\text{cytokine,} i}^\text{eff} = \sigma_{\text{cytokine,} i} \cdot \frac{K_F^\text{immune}}{K_F^\text{immune} + F} \tag{2}\]
where the cortisol concentration \(F\) inhibits pro-inflammatory cytokine production. The reduced cortisol output in ME/CFS (HPA Axis Models) weakens this inhibitory brake, permitting higher cytokine levels than would otherwise be sustained. This neuroimmune coupling explains why stress management—by reducing cortisol demand—and low-dose hydrocortisone replacement have shown limited benefit in some ME/CFS patients.
3 Vagal Signaling
The vagus nerve provides a rapid bidirectional communication pathway between the periphery and the CNS. Afferent vagal fibers detect peripheral cytokines (particularly IL-1\(\beta\) and TNF-\(\alpha\)) and transmit inflammatory signals to the nucleus tractus solitarius (NTS) (Marty et al. 2008), producing sickness behavior (fatigue, anorexia, social withdrawal). Efferent vagal fibers mediate the cholinergic anti-inflammatory pathway, suppressing macrophage TNF-\(\alpha\) production through \(\alpha\) 7 nicotinic acetylcholine receptors (Huerta, Masters, and Matheny 2025). The vagal coupling is modeled as:
\[ \sigma_\text{TNF}^\text{eff} = \sigma_\text{TNF} \cdot \frac{K_\text{vagal}}{K_\text{vagal} + V_\text{efferent}} \tag{3}\]
where \(V_\text{efferent}\) is vagal efferent tone from the ANS model (ans balance). Reduced vagal tone in ME/CFS therefore diminishes the cholinergic anti-inflammatory pathway, providing an additional mechanism for sustained peripheral inflammation.