Small-Animal PET/CT Imaging of Local and Systemic Immune Response Using ⁶⁴Cu-αCD11b.

Abstract

Current noninvasive imaging methods for monitoring immune response were largely developed for interrogation of the local reaction. This study developed the radiotracer ⁶⁴Cu-labeled anti-CD11b (⁶⁴Cu-αCD11b) for longitudinal assessment of local and systemic immune response involving mobilization of CD11b⁺ myeloid cells by small-animal PET/CT. <b>Methods:</b> Acute or chronic inflammation in the ears of BALB/c mice was induced by 12-<i>o</i>-tetradecanoylphorbol-13-acetate. Acute lung inflammation was induced by intratracheal lipopolysaccharide inoculation. αCD11b was conjugated with <i>p</i>-SCN-Bn-DOTA followed by labeling with ⁶⁴Cu. PET/CT and biodistribution were evaluated at different times after intravenous injection of ⁶⁴Cu-αCD11b. Cell populations from bone marrow (BM) and spleen were analyzed by flow cytometry. <b>Results:</b>⁶⁴Cu-αCD11b was primarily taken up by BM and spleen in control mice. In comparison, ⁶⁴Cu-αCD11b uptake was significantly reduced in the BM and spleen of CD11b-knockout mice, indicating that ⁶⁴Cu-αCD11b selectively homed to CD11b⁺ myeloid cells in vivo. In mice with ear inflammation, for the local inflammatory response, ⁶⁴Cu-αCD11b PET/CT revealed significantly higher ⁶⁴Cu-αCD11b uptake in the inflamed ears in the acute inflammation phase than the chronic phase, consistent with markedly increased infiltration of CD11b⁺ cells into the inflammatory lesions at the acute phase. Moreover, imaging of ⁶⁴Cu-αCD11b also showed the difference in mouse systemic response for different inflammatory stages. Compared with uptake in control mice, BM ⁶⁴Cu-αCD11b uptake in mice with ear inflammation was significantly lower in the acute phase and higher in the chronic phase, reflecting an initial mobilization of CD11b⁺ cells from the BM to the inflammatory foci followed by a compensatory regeneration of CD11b⁺ myeloid cells in the BM. Similarly, in mice with lung inflammation, ⁶⁴Cu-αCD11b PET/CT readily detected acute lung inflammation and recruitment of CD11b⁺ myeloid cells from the BM. Immunohistochemistry staining and flow cytometry results confirmed the noninvasive imaging of PET/CT. <b>Conclusion:</b>⁶⁴Cu-αCD11b PET/CT successfully tracked ear and pulmonary inflammation in mice and differentiated acute from chronic inflammation at the local and systemic levels. ⁶⁴Cu-αCD11b PET/CT is a robust quantitative method for imaging of local and systemic immune responses.