Expression of α₁β₁ on CD11b⁺ cells is restricted to monocytes/macrophages, and in vivo blockade of α₁β₁ results in decreased number and activation state of monocytes/macrophages infiltrating the lamina propria. Colonic cross sections from WT mice that were treated for 7 days with DSS were stained with a combination of directly labeled mAb’s: (a) FITC anti-CD11b and PE anti-Gr1 mAb’s; (b) Alexa 488 anti-α₁ and PE anti-Gr1 mAb’s; (c) Alexa 488 anti-α₁ and PE anti-CD11b mAb’s; (d) Alexa 488 anti-α₁ and PE anti-F4/80 mAb’s. Expression of α₁β₁ on CD11b⁺ cells was restricted to monocytes and macrophages as judged by its colocalization on F4/80⁺ and CD11b⁺Gr1^– cells. Gr1 and F4/80 are granulocyte and monocyte/macrophage markers, respectively. The effect of in vivo α₁β₁ blockade on the number and activation state of the monocytes/macrophages infiltrating the lamina propria was quantitated by dual-color immunohistochemical analysis using F4/80 and CD14 mAb’s (e). The number of F4/80⁺ (black bars) cells per hpf and the percentage of CD14⁺ (white bars) F4/80 cells infiltrating the lamina propria following various treatment regimens (outlined in legends for Figure 1 and Figure 3) was quantitated. Data are expressed as number of F4/80+ cells per hpf and percentage of F4/80+ cells expressing the CD14 activation marker, mean ± SEM. In a–d, magnification is ×200 and bar represents 50 μm.