Small-animal PET/CT imaging of local and systemic immune response using ⁶⁴Cu-ACD11B

Current noninvasive imaging methods for monitoring immune response were largely developed for interrogation of the local reaction. This study developed the radiotracer ⁶⁴Cu-labeled antiCD11b (⁶⁴Cu-αCD11b) for longitudinal assessment of local and systemic immune response involving mobilization of CD11b¹ myeloid cells by small-animal PET/CT. Methods: Acute or chronic inflammation in the ears of BALB/c mice was induced by 12-o-tetradeca-noylphorbol-13-acetate. Acute lung inflammation was induced by intratracheal lipopolysaccharide inoculation. αCD11b was conjugated with p-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. Results: 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 CD11b¹ 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 CD11b¹ 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 CD11b1 cells from the BM to the inflammatory foci followed by a compensatory regeneration of CD11b¹ myeloid cells in the BM. Similarly, in mice with lung inflammation, ⁶⁴Cu-αCD11b PET/CT readily detected acute lung inflammation and recruitment of CD11b¹ myeloid cells from the BM. Immunohistochemistry staining and flow cytometry results confirmed the noninvasive imaging of PET/CT. Conclusion: 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.