Noninvasive PET Detection of CD69-Positive Immune Cells Before Signs of Clinical Disease in Inflammatory Arthritis

Rheumatoid arthritis (RA) is the most common inflammatory joint disease, and early diagnosis is key for effective disease management. CD69 is one of the earliest cell surface markers seen at the surface of activated immune cells, and CD69 is upregulated in synovial tissue in patients with active RA. In this study, we evaluated the performance of a CD69-targeting PET agent, [⁶⁸Ga]Ga-DOTA-Z_CAM241, for early disease detection in a model of inflammatory arthritis. Methods: A model of inflammatory arthritis was induced by transferring splenocytes from KRN T-cell receptor transgenic B6 mice into T-cell-deficient I-A^g7 major histocompatibility complex class II-expressing recipient mice. The mice were examined longitudinally by [⁶⁸Ga]Ga-DOTA-Z_CAM241 PET/CT before and 3, 7, and 12 d after induction of arthritis. Disease progression was monitored by clinical parameters, including measuring body weight and scoring the swelling of the paws. The uptake of [⁶⁸Ga]Ga-DOTA-Z_CAM241 in the paws was analyzed and expressed as SUV_mean Tissue biopsy samples were analyzed for CD69 expression by flow cytometry or immunostaining for a histologic correlate. A second group of mice was examined by a nonbinding, size-matched Affibody molecule as the control. Results: Clinical symptoms appeared 5-7 d after induction of arthritis. The uptake of [⁶⁸Ga]Ga-DOTA-Z_CAM241 in the joints was negligible at baseline but increased gradually after disease induction. An elevated PET signal was found on day 3, before the appearance of clinical symptoms. The uptake of [⁶⁸Ga]Ga-DOTA-Z_CAM241 correlated with the clinical score and disease severity. The presence of CD69-positive cells in the joints and lymph nodes was confirmed by flow cytometry and immunostaining. The uptake of the nonbinding tracer that was the negative control also increased gradually with disease progression, although to a lesser extent than with [⁶⁸Ga]Ga-DOTA-Z_CAM241 Conclusion: The uptake of [⁶⁸Ga]Ga-DOTA-Z_CAM241 in the inflamed joints preceded the clinical symptoms in the KRN T-cell transfer model of inflammatory arthritis, in accordance with immunostaining for CD69. [⁶⁸Ga]Ga-DOTA-Z_CAM241 is thus a promising PET imaging marker of activated immune cells in tissue during RA onset.