triggers expression of CD20 on otherwise non-expressing multiple
myeloma cells and may set them up for destruction by anti- CD20
monoclonal antibodies such as rituximab (Rituxan), Steven P. Treon,
MD, PhD, of Dana-Farber Cancer Institute reported at the ASH meeting.
Rituximab is an appealing therapeutic agent because of its relative
lack of toxicity, but other studies have shown that in multiple
myeloma, it is effective primarily in patients with CD20+ cells.
The mechanism of CD20 induction by IFN-g is related to
upregulation of PU.1, a transactivator of CD20 which is down-regulated
with B-cell differentiation to plasma cells, Dr. Treon
explained. IFN-g is also attractive
because it has single-agent activity in multiple myeloma. IFN-g
has antiproliferative and cytotoxic effects on multiple myeloma cell
lines and on fresh multiple myeloma patient bone marrow plasma
cells, Dr. Treon continued. In phase I/II clinical
trials, response rates up to 15% have been reported with use of IFN-g
in relapsed multiple myeloma patients.
Changes in CD20
To test whether dosing multiple myeloma cells with IFN-g
would upregulate CD20 expression and whether this would translate
into increased binding of rituximab by the treated cells, researchers
set up cell cultures. These included plasma cells from 20 multiple
myeloma patients, B cells from 9 multiple myeloma patients, B cells
from 5 chronic lymphocytic leukemia patients, B-cells from 11 normal
donors, plasma cells from 3 normal donors, and CD34+ hematopoietic
progenitor cells from 5 normal donors. Each type of cell was cultured
with and without IFN-g at 1 to 100
µ/mL for 48 hours. Changes in CD20 expression were evaluated
with multicolor flow cytometry.
Binding of rituximab to cells from a multiple myeloma cell line (RPMI
8226) and to plasma cells from multiple myeloma patients was also
measured before and after treatment with IFN-g.
Rituximab Binding Increased
Dr. Treon reported that these studies demonstrated that IFN-g
increases the number of cells that express CD20 and increases the
intensity of CD20 expression by individual cells in cultures of
multiple myeloma plasma cells, multiple myeloma B cells, and normal
donor plasma cells. IFN-g did not change
CD20 expression in the other cells tested. The changes in CD20
expression correlated with expression of PU.1. Interestingly, the
levels of IFN-g receptor expression did not correlate with these
differences in effect on CD20 expression.
Rituximab binding to the multiple myeloma cell line and to cultured
plasma cells from multiple myeloma patients increased following
culture with IFN-g. IFN-g
upregulates CD20 expression on multiple myeloma patient B cells and
on multiple myeloma patient plasma cells, Dr. Treon noted.
IFN-g increases rituximab binding to multiple myeloma plasma
cells. IFN-g increases Fc receptor
expression in natural killer (NK) cells and monocytes and augments NK
and monocyte ADCC activity, Dr. Treon added.
Induction of CD20
IFN-g selectively induces CD20
expression on multiple myeloma B cells and plasma cells at
concentrations that are achievable pharmacodynamically and
facilitates binding of rituximab. Induction of CD20 on multiple
myeloma cells occurs as early as 6 hours and maximizes at 24 hours
following culture with IFN-g. Sequential
administration of IFN-g on day 1 and 3
provided for maximal induction of CD20.
These data provide a rationale for combining IFN-g
with CD20-directed serotherapies for multiple myeloma. Dr. Treon said
that the researchers are considering conducting a combination IFN-g/rituximab
study in multiple myeloma patients.