Considering the prospects for different proposed mechanisms
Dr. Coles shared some encouraging news at ACTRIMS Forum 2018. His team and other researchers are getting closer to understanding the cellular mechanism underlying the paradoxical autoimmunity associated with alemtuzumab. Published reports in the literature from others suggest faulty immune B cells could be the culprit, pointing to a similar reconstitution of B cells after bone marrow transplantation. However, he said, “There is no difference in this reconstitution pattern between those who do and don’t get autoimmunity. So we do not think that autoimmunity after alemtuzumab is primarily a B cell problem.”
Other investigators have pointed to possible depletion of a key immune regulatory cell associated with alemtuzumab, such as alterations in CD52-positive T cells that cause depletion in T cells as part of an autoimmune cascade that involve CD52-high expressing cells and sialic acid-binding immunoglobulin-like lectin 10. “I’m not going to describe why we don’t believe any of this,” Dr. Coles said, but added, “We cannot replicate the data in type 1 diabetes or MS about the depletion of T cells.”
Along with his colleague Joanne Jones, PhD, a clinical fellow in the same department at the University of Cambridge, Dr. Coles and his team instead propose that autoimmunity after alemtuzumab therapy is associated with a homeostatic proliferation of T cells in the context of a defective thymus. “We see thymic function reduced after alemtuzumab for a few months. We don’t know if alemtuzumab is having a direct impact on the thymus or if it’s an indirect effect though a cytokine storm at the time of administering alemtuzumab.”
In addition, in contrast to B cells, both CD4-positive and CD8-positive T cells are clonally restricted after alemtuzumab treatment, Dr. Coles explained.
“These are the only changes that distinguish patients who do and do not develop autoimmunity,” he said. “Those who develop autoimmunity have reduced clonality and have impaired thymic function compared to those who don’t.”
As the theory goes, the limited clonal repertoire leads to expansion of the T cells, preferentially expanding autoreactive T cells, leading to B-cell- and antibody-mediated autoimmunity.