Immunohistochemistry using monoclonal antibody OPD4 demonstrating memory (CD45RO⁺) CD4⁺ T cells in the jejunum of an uninfected neonate (21 days old, DP53) and age-matched SIV-infected neonate (21 days after infection, EC73). Note that numerous memory CD4⁺ T cells are evident in the uninfected jejunum (A) as compared with the SIV-infected neonate (B). (C) Fluorescent in situ hybridization for SIV combined with immunofluorescence for OPD4 (CD4⁺ memory cells) in the intestine of 3 SIV-infected neonates examined 12 to 21 days after SIV infection. Essentially all of the SIV-infected cells (red) are CD4⁺CD45RO⁺ memory cells as defined by reactivity with monoclonal antibody OPD4 (green) using confocal microscopy. (D) Comparison of absolute numbers of CD4⁺CD45RO⁺ cells per square millimeter of jejunum lamina propria between SIV-infected and uninfected (control) neonates. Memory CD4⁺ T cells are markedly decreased in the intestine of all neonates as early as 12 days after SIV infection. Panels A and B were photographed using a Leica DM LB microscope (Leica Microsystems, Bannockburn, IL) with an N-planar 20×/0.40 NA objective equipped with a Spot 3.2.0 digital camera (Diagnostic Instruments, Sterling Heights, MI). Panel C was taken with a Leica TCS SP2 confocal microscope (Leica Microsystems, Exton, PA). NIH image version 1.62 (NIH, Gaithersburg, MD) and Adobe Photoshop (Adobe Systems, San Jose, CA) were used to assign colors to the images.