Purpose: Glioblastoma is a lethal brain tumor, heavily infiltrated by tumor-associated myeloid cells (TAMCs). TAMCs are emerging as a promising therapeutic target as they suppress anti-tumor immune responses and promote tumor cell growth. Quantifying TAMCs using non-invasive immunoPET could facilitate patient stratification for TAMC-targeted treatments and monitoring of treatment efficacy. As TAMCs uniformly express the cell surface marker, integrin CD11b, we evaluated a Zr-89 labeled anti-CD11b antibody for non-invasive imaging of TAMCs in a syngeneic orthotopic mouse glioma model.
Procedures: A human/mouse cross-reactive anti-CD11b antibody (clone M1/70) was conjugated to a DFO chelator and radiolabeled with Zr-89. PET/CT and biodistribution with or without a blocking dose of anti-CD11b Ab were performed 72 h post-injection (p.i.) of [89Zr]anti-CD11b Ab in mice bearing established orthotopic syngeneic GL261 gliomas and in non tumor-bearing mice. Flow cytometry and immunohistochemistry of dissected GL261 tumors were conducted to confirm the presence of CD11b+ TAMCs.
Results: Significant uptake of [89Zr]anti-CD11b Ab was detected at the tumor site (SUVmean = 2.60 ± 0.24) compared with the contralateral hemisphere (SUVmean = 0.6 ± 0.11). Blocking with a 10-fold lower specific activity of [89Zr]anti-CD11b Ab markedly reduced the SUV in the right brain (SUVmean = 0.11 ± 0.06), demonstrating specificity. Spleen and lymph nodes (myeloid cell rich organs) also showed high uptake of the tracer, and biodistribution analysis correlated with the imaging results. CD11b expression within the tumor was validated using flow cytometry and immunohistochemistry, which showed high CD11b expression primarily in the tumoral hemisphere compared with the contralateral hemisphere with very minimal accumulation in non tumor-bearing brain.
Conclusion: These data establish that [89Zr]anti-CD11b Ab immunoPET targets CD11b+ cells (TAMCs) with high specificity in a mouse model of GBM, demonstrating the potential for non-invasive quantification of tumor-infiltrating CD11b+ immune cells during disease progression and immunotherapy in patients with GBM.
Keywords: Glioblastoma (GBM); Myeloid-derived suppressor cells (MDSCs); Positron-emission tomography (PET); Tumor-associated macrophages (TAMs); Tumor-associated myeloid cells (TAMCs).