Breast Milk Pro-Inflammatory Cytokines Are Suppressed During Infant Infection and Show Heterogeneous Recovery in Convalescence

Breast milk plays a pivotal role in maternal-infant immune communication. We examined cytokine dynamics in 45 asymptomatic breastfeeding mothers: 29 nursing infants with acute infection and 16 nursing healthy infants. All infants were born at full term, with high Apgar scores, and the mothers had no clinical or laboratory signs of inflammation. Milk samples were collected 3–5 days after the onset of infant illness and 4–6 weeks later during convalescence. Cytokine concentrations (IFN‑γ, IL‑1β, IL‑17A, IL‑4, IL‑6, and TNF‑α) were measured in the aqueous phase of breast milk using Human HS ProcartaPlex™ Mix&Match assay. Only IL‑6, IL‑1β, and TNF were consistently detectable. In the mothers of ill infants, pro‑inflammatory cytokines, particularly IL‑6, were markedly suppressed during acute infection and correlated with elevated infant C‑reactive protein levels. Cytokine recovery during convalescence was heterogeneous: IL‑6 rebounded in younger mothers and in infants with milder leukocytosis, but recovery was blunted in older mothers and in cases of pronounced infant inflammation. TNF restoration was more frequent in early lactation, suggesting an influence of lactation stage. These findings reveal that breast milk cytokine responses are dynamically regulated by both infant inflammatory status and maternal characteristics, highlighting an adaptive mechanism of immune communication during lactation.