Innovative PET imaging identifies CD4+ T cells causing intestinal inflammation

In a murine model of colitis, anti-CD4 immuno-positron emission tomography (PET) was able to detect CD4+ T cells in inflamed colons and enlarge mesenteric lymph nodes (MLNs), according to researchers from the University of California, Los Angeles.

In the colons of colitic mice, previous research has confirmed an increased number of CD4+ T cells using anti-CD4 immunohistochemistry.

In this investigation, researchers were able to use anti-CD4 immuno-PET imaging with GK1.5 cys-diabody (cDb), an anti-mouse CD4 antibody fragment derived from GK 1.5 hybridoma, to detect CD4+ T cells.

The findings “should provide a useful tool for investigating CD4+ T cells in murine models of inflammatory bowel disease (IBD) and other diseases with a CD4 inflammatory component,” they claimed in The Journal of Nuclear Medicine.

“CD4+ T cells play a key role in human IBD and identifying them throughout the bowel could be useful to monitor disease severity and predict treatment success. Imaging is less intrusive than colonoscopies, which are uncomfortable for patients and carry a risk of bowel perforation; furthermore, inflammation can vary depending on the position within the bowel,” wrote these researchers, led by Amanda C. Freise, BSc, Crump Institute for Molecular Imaging, Department of Molecular and Medical Pharmacology, David Geffen School of Medicine at UCLA, and colleagues.

IBD in humans is characterized, in part, by aberrant CD4+ T-cell responses. Identifying foci of inflammation within the gut has traditionally required invasive procedures such as colonoscopy and biopsy.

¹⁸F-FDG-PET imaging can successfully identify inflammation with high sensitivity and specificity, but uptake of ¹⁸F-FDG-PET in the gut is highly variable, noted the investigators, making its use less than ideal for a screening tool. Freise and colleagues, therefore, investigated molecular imaging with GK1.5 cDb in a dextran sulfate sodium (DSS) mouse model of IBD.

Researchers conjugated GK1.5 cDb to malDFO, and radiolabeled this with 89Zr. They injected ⁸⁹Zr-malDFO-GK1.5 cDb into mice, separated into groups of four. ⁸⁹Zr-malDFO-GK1.5 cDb successfully detected CD4+ T cells in the colons and MLNs, and the colons of colitic mice showed uptake of ⁸⁹Zr-malDFO-GK1.5 cDb. Researchers observed little to no uptake of ⁸⁹Zr-malDFO-GK1.5 cDb, however, in the colons of mice without colitis. Quantification of the distal colon region with region of interest analysis showed that uptake significantly increased in colitic mice.

“This result is consistent with other studies, which report that the most severe damage in the DSS model occurs in that region,” the investigators wrote.

Anti-CD4 immunohistochemical staining showed increased infiltration of CD4+ T cells into the colons of DSS-treated mice.

Significantly increased uptake of ⁸⁹Zr-maleimide-deferoxamine-GK1.5 cDb in the distal colon of colitic mice was visible in vivo in PET scans, and region of-interest analysis of the distal colon confirmed increased activity in DSS mice. Ex vivo scans confirmed the results of the in vivo scans.

“Interestingly, uptake was also increased in the ceca of colitic mice, although we did not observe increased CD4+ T cells by immunohistochemistry in colitic ceca; this may be due at least in part to enhanced permeability of the cecum due to DSS-induced inflammation,” the authors wrote.

In DSS-treated mice, MLNs from colitic mice were significantly enlarged and visible in PET images but the percentage of CD4+ T cells decreased, “which suggests that both CD4+ T cells and other types of immune cells were present in higher total numbers, resulting in increased size but not increased concentration.”

These findings suggest that identifying CD4+ T cells through the bowel may be useful in monitoring IBD disease severity as well as in predicting treatment success, concluded these authors. Furthermore, the less invasive nature of imaging compared with colonscopy may benefit patients as well.