Immunological contributions to age-dependent variations in behavioural responses to cutaneous inflammation

Systemic responses to immune challenge are immature at birth. However exposure to experimental inflammogens are able to produce an immunologic response which is characterised by swelling and oedema but, unlike in adults, does not result in sensory hypersensitivity. We sought to investigate whether the lack of nociceptive hypersensitivity was as a result of altered hemapoietic immune cell recruitment to the site of inflammation and/or differences in the cytokine and chemokine profile released by tissue invading cells. Postnatal (day of birth) and young adult (40-days old) Sprague-Dawley rats were used. Inflammation was induced by s.c. injection of Complete Freunds Adjuvant (CFA) unilaterally into the one hind paw. Mechanical withdrawal thresholds were measured before and after injection (2-168hrs). In adults a significant hyperalgesia was evoked which was absent in neonates. Immunohistochemical analysis of invading immune cells present in the perfusion fixed skin showed that although total cell numbers in the paw were the same in both age groups, neonates recruited more cells positive for both cell surface markers CD68 and Mannose-receptor (MR) whereas adults recruited significantly more cells positive for MR alone. There were no differences in neutrophil recruitment (as measured with H&E staining). TaqMan qPCR demonstrated that the temporal profile of cytokine production in the skin differed between ages with neonates responding faster than adults and that neonates produced significantly more IL-1b and IL-27 then adults who expressed significantly more IL-6 and IL-10. This study illustrates that in neonates the cell recruitment and cytokine profiles are markedly different to those seen in adults; this may in part explain why behavioural responses to inflammation are suppressed relative to adults.