UCLA

CD4+ T cells orchestrate the immune system and play a key role in the response to infection, the induction and maintenance of some autoimmune diseases, and immunotherapy of cancer. Comprehensive, noninvasive imaging of CD4+ T cells would be an improvement upon current methods of analysis such as blood draws and biopsies. Molecular imaging of CD4+ T cells can be accomplished with immuno-positron emission tomography (PET), which utilizes antibody-based probes to detect and quantify cell surface markers. This dissertation describes the characterization of the functional properties of GK1.5 cys-diabody (cDb), an anti-mouse CD4 antibody fragment, and its use in detecting CD4+ T cells with immunoPET in a mouse model of inflammatory bowel disease.

Ideally, imaging probes should have minimal biological effect on the target of interest, and be suitable for imaging at low doses. In this work, the effect of GK1.5 cDb on murine CD4+ T cell function and CD4 expression was explored at multiple doses. Furthermore, the effect of protein dose on image contrast was assessed by performing a dose-esclation immunoPET study and ex vivo biodistribution analysis. The lowest protein dose of 2 �g 89Zr-labeled GK1.5 cDb resulted in improved contrast of lymphoid organs in PET scans compared to higher doses. Intravenous administration of GK1.5 cDb caused a dose-dependent, transient decrease in CD4 expression in blood and lymphoid organs, which recovered within 3 days post-injection. Proliferation was inhibited in vivo in inguinal lymph nodes but not the spleen by injection of 40 �g GK1.5 cDb. Overall, using low dose GK1.5 cDb minimized biological effects on CD4+ T cells.

A potential application of anti-CD4 immunoPET is investigation of the localization of CD4+ T cells at sites of inflammation, and use imaging to guide treatment decisions. Here, immunoPET imaging with 89Zr-malDFO-GK1.5 cDb was used to image the distribution of CD4+ T cells in the intestinal region of mice treated with dextran sulfate sodium to induce colitis. Increased uptake of GK1.5 cDb in the distal colon and mesenteric lymph nodes (MLN) of colitic mice was apparent from in vivo scans. Ex vivo scans and biodistribution analysis confirmed higher uptake in DSS-treated colons, ceca, and MLN. Anti-CD4 immunoPET was also used to image colitic mice after immunosuppressive treatment with cyclosporin A (CsA), although no significant changes were seen in the biodistribution of the probe between treated and non-treated mice.

This work shows that low dose GK1.5 cDb yields high-contrast immunoPET images with minimal effects on T cell biology, and was a useful tool for investigating CD4+ T cells in the context of preclinical inflammatory bowel disease.