Aldehyde dehydrogenase (ALDH) activity is locally decreased during high-dose T. muris infection. Lamina propria leukocytes (LPLs) were isolated from the large intestines of susceptible AKR mice and resistant C57BL/6 mice at weekly intervals throughout the course of T. muris infection. (a) Gating strategy for 7AAD-CD45+MHCII+CD103-F4/80+CD11b+ macrophages and 7AAD-CD45+MHCII+CD103+F4/80-CD11c+ dendritic cells that were positively stained for ALDH activity. The blue line represents ALDH+ cells and the red line shows ALDH+ activity incubated with the retinaldehyde dehydrogenase inhibitor, diethylaminobenzaldehyde (DEAB), as a negative control. Percentage of MHCII+CD103-F4/80+CD11b+ macrophages or MHCII+CD103+F4/80-CD11c+ dendritic cells that have ALDH activity at each stage of infection in AKR mice given a high-dose (chronic) infection (b), C57BL/6 mice given a high-dose (acute) infection (c) and C57BL/6 mice given a low-dose (chronic) infection (d). Graphs show means + SEM. (n = 3–4) *P < 0·05, **P < 0·01, ***P < 0·001, compared to naïve levels, unless otherwise shown. Data is representative of two independent experiments, repeated at key time points.