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The mechanism underlying the dose effect of probiotics on ameliorating diarrhea has not been fully elucidated.Here, low (1×109 CFU/mL) or high (1×1011 CFU/mL) doses ofLactobacillus rhamnosus ATCC 7469 were administered orally to piglets for 1 week before F4 (K88)-positive enterotoxigenic Escherichia coli (F4+ ETEC) challenge.Administration of a low, but not a high, dose of L.rhamnosus decreased the percentage of CD3+ CD4+ CD8-T cells in the peripheral blood.Notably, transiently increased serum concentrations of interleukin-17A (IL-17A) were observed after F4+ ETEC challenge in pigs pretreated with a high dose of L.rhamnosus.Administration of L.rhamnosus increased the percentage of the small intestinal lamina propria CD3+ CD4+ CD8-cells and Peyers patch CD3+ CD4-CD8-and CD3-CD4-CD8+ cells.The percentage of ileal intraepithelial CD3+ CD4-CD8+ cells increased only in the high-dose piglets.Administration of L.rhamnosus downregulated expression of ileal IL-17A after F4+ ETEC challenge but had no effect on expression of gamma interferon (IFN-γ), IL-12, IL-4, and FOXP3 mRNA in the small intestine.Expression of jejunal IL-2, ileal transforming growth factor β1 (TGF-β1), and ileal IL-10 was upregulated in the low-dose piglets after F4+ ETEC challenge.Our findings suggest that amelioration of infectious diarrhea in piglets by L.rhamnosus is associated with the generation of lamina propria CD3+ CD4+ CD8-T cells, the expansion of Peyers patch CD3+ CD4-CD8-and CD3-CD4-CD8+ cells, and the attenuation of F4+ ETEC-induced increase in CD3+ CD4+ CD8+ T cells in the small intestine.However, consumption of high doses of L.rhamnosus may increase levels of serum IL-17A after F4+ ETEC challenge, thus eliciting a strong proinflammatory response.