B lymphocytes are a key pathological feature of multiple sclerosis (MS), and are becoming an important therapeutic target for this condition. Currently, there is no approved technique to non-invasively visualize B cells in the central nervous system (CNS) to monitor MS disease progression and response to therapies. Here we evaluated 64Cu-Rituximab, a radiolabeled antibody specifically targeting the human B cell marker CD20, for its ability to image B cells in a mouse model of MS using positron emission tomography (PET).
James ML, Hoehne A, Mayer AT, Lechtenberg K, Moreno M, Gowrishankar G, Ilovich O, Natarajan A, Johnson EM, Nguyen J, Quach L, Han M, Buckwalter M, Chandra S, Gambhir SS. Imaging B cells in a mouse model of multiple sclerosis using 64Cu-Rituximab-PET. J Nucl Med. 2017 Jul 7. pii: jnumed.117.189597. doi: 10.2967/jnumed.117.189597. [Epub ahead of print]
Methods: To model CNS infiltration by B cells, experimental autoimmune encephalomyelitis (EAE) was induced in transgenic mice that express human CD20 on B cells. EAE mice were given subcutaneous injections of Myelin Oligodendrocyte Glycoprotein fragment 1-125 (MOG1-125) emulsified in complete Freund’s adjuvant. Control mice received complete Freund’s adjuvant alone. PET imaging of EAE and control mice was performed 1, 4, and 19h following 64Cu- (Radioactive copper sixty four) Rituximab administration. Mice were perfused and sacrificed after final PET scan, and radioactivity in dissected tissues was measured with a gamma-counter. CNS tissues from these mice were immunostained to quantify B cells or further analyzed via digital autoradiography. Results: Lumbar spinal cord PET signal was significantly higher in EAE mice compared to controls at all evaluated time points (e.g., 1h post-injection: 5.44 ± 0.37 vs. 3.33 ± 0.20 %ID/g, p<0.05). 64Cu-Rituximab-PET signal in brain regions ranged between 1.74 ± 0.11 and 2.93 ± 0.15 %ID/g for EAE mice compared to 1.25±0.08 and 2.24±0.11%ID/g for controls, p<0.05 for all regions except striatum and thalamus at 1h post-injection. Similarly, ex vivo bio-distribution results revealed notably higher 64Cu-Rituximab uptake in brain and spinal cord of huCD20tg EAE, and B220 immunostaining verified that increased 64Cu-Rituximab uptake in CNS tissues corresponded with elevated B cells.
Conclusion: B cells can be detected in the CNS of EAE mice using 64Cu-Rituximab-PET. Results from these studies warrant further investigation of 64Cu-Rituximab in EAE models and consideration of use in MS patients to evaluate its potential for detecting and monitoring B cells in the progression and treatment of this disease. These results represent an initial step toward generating a platform to evaluate B cell-targeted therapeutics en route to the clinic.
Note the red in the spinal cord (White Arrow)
As we enter the age of the B cells, we would like to assess the effects of treatments on B cells. This is a study in mice and uses a radioactive version of rituximab to do this. In EAE they can pick up B cells in mice with EAE. This was easy peasey, but can this approach be used in humans.
In the animals, they are giving their imaging agents at a time when they will get into the CNS. The question is what happens when there are no focal lesions one images the penetration of antibody will not be great. It’s a start.