Monoclonal antibodies

Photo by Linda Bartlett

Results of preclinical research help explain how antitumor monoclonal antibodies (mAbs) fight lymphoma.

Researchers uncovered a 2-step process that revolves around 2 antibody-binding receptors found on different types of immune cells.

Experiments suggested that these Fc receptors are needed to eradicate lymphoma and ensure it doesn’t return.

The researchers reported these findings in an article published in Cell.

“These findings suggests ways current anticancer antibody treatments might be improved, as well as combined with other immune-system-stimulating therapies to help cancer patients,” said study author Jeffrey Ravetch, MD, PhD, of The Rockefeller University in New York, New York.

Previous research has shown that antitumor mAbs bind to Fc receptors on activated immune cells, prompting those immune cells to kill tumors.

However, it was unclear which Fc receptors are involved or how the tumor killing led the immune system to generate memory T cells against these same antigens, in case the tumor producing them should return.

Dr Ravetch and David DiLillo, PhD, also of The Rockefeller University, investigated this process by injecting CD20-expressing lymphoma cells into mice with immune systems engineered to contain human Fc receptors, treating the mice with anti-CD20 mAbs, and then re-introducing lymphoma.

Wild-type C57BL/6 mice received syngeneic EL4 lymphoma cells expressing human CD20 (EL4-hCD20 cells). When these mice received treatment with an mIgG2a isotype anti-hCD20 mAb, they all survived.

Ninety days later, after the mAb had been cleared from their systems, the mice were re-challenged with EL4-hCD20 tumor cells, at a dose 10-fold greater than the initial challenge, but they did not receive any additional mAb.

All of these mice survived, but tumor/mAb-primed mice that were re-challenged with EL4-wild-type cells, which don’t express hCD20, had poor survival. Results were similar with a different anti-hCD20 mAb, clone 2B8.

The researchers also re-challenged tumor/mAb-primed mice with B6BL lymphoma cells that expressed either hCD20 or an irrelevant antigen, mCD20. All of the mice re-challenged with B6BL-mCD20 cells had died by day 31, but 80% of the mice re-challenged with B6BL-hCD20 cells survived at least 90 days.

Drs Ravetch and DiLillo said these results suggest an anti-hCD20 immune response is generated after the initial FcγR-mediated clearance of tumor cells by antibody-dependent cellular cytotoxicity.

The researchers then took a closer look at the role of Fc receptors, keeping in mind that different types of immune cells can express different receptors.

Based on the cells the researchers thought were involved, they looked to the Fc receptors expressed by cytotoxic immune cells that carried out the initial attack on tumors, as well as the Fc receptors found on dendritic cells, which are crucial to the formation of memory T cells.

To test the involvement of these receptors, the pair altered the mAbs delivered to the lymphoma-infected mice so as to change their affinity for the receptors. Then, they looked for changes in the survival rate of the mice after the first and second challenges with lymphoma cells.

When they dissected this process, the researchers uncovered 2 steps. The Fc receptor FcγRIIIA, which is found on macrophages, responded to mAbs and prompted the macrophages to engulf and destroy the antibody-laden tumor cells.

These same antibodies, still attached to tumor antigens, activated a second receptor, FcγRIIA, on dendritic cells, which used the antigen to prime T cells. The result was the generation of a T-cell memory response that protected the mice against future lymphoma cells expressing CD20.

“By engineering the antibodies so as to increase their affinity for both FcγRIIIA and FcγRIIA, we were able to optimize both steps in this process,” Dr DiLillo said.

“Current antibody therapies are only engineered to improve the immediate killing of tumor cells but not the formation of immunological memory. We are proposing that an ideal antibody therapy would be engineered to take full advantage of both steps.”

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