| Abstract |
Attachment to membranes via a glycosylphosphatidylinositol (GPI) anchor as
an alternative to the use of a transmembrane region has been described
for a large number of membrane proteins including the complement control
proteins decay-accelerating factor (CD55) and membrane inhibitor of reactive
lysis (CD59) as well as the cell surface molecules CD48 and leucocyte
function antigen-3 (CD58). Those structures have been demonstrated to be
missing from the membrane of the abnormal blood cells in patients with
paroxysmal nocturnal hemoglobinuria (PNH). Here we report on a similar
defect for GPI-anchored membrane proteins on a new human myeloma cell line
designated LOPRA-2 with the phenotype of terminally differentiated B cells
derived from peripheral blood of a patient with plasma cell leukemia. The
cultured cells secrete immunoglobulin molecules consisting of gamma1 heavy
and kappa light chains; they express the plasma cell antigen PCA-1, the
antigens CD28 (Kolt-2), CD38 (OKT10), CD56 (Leu19), CD71 (OKT9) and some
epitopes of the CD24 antigen (HB8, V1B E3), but are negative for surface
immunoglobulins, HLA class II antigens (HLA-DP, -DQ, -DR) and other B-cell
markers such as CD10 (CALLA), CD19 (B4), CD20 (B1), CD21 (B2), CD22 (HD39),
CD23 (MHM6), CD37 (BL14), and CD39 (G28-8) as analyzed by both flow
cytometry and immunocytochemistry (APAAP). Surprisingly, the cultured
myoloma cells did not express the GPI-linked complement regulatory proteins
CD55 and CD59 as well as the GPI-anchored cell surface molecules CD48 and
CD58, which were found present on all other established myeloma cell lines
tested (LOPRA-1, U266, ARH-77, RPMI 8226). The GPI-anchored molecules
could not be induced by in vitro treatment of LOPRA-2 cells with various
recombinant cytokines (IFN alpha, IFN gamma, TNF alpha, G-CSF, GM-CSF, IL1
to IL8) or differentiation-inducing agents (TPA. retinoids). On the patient's
peripheral rod blood cells and neutrophils no defect for GPI-anchored
proteins could be detected. In addition, the neoplastic plasma cells
freshly isolated from the patient's peripheral blood contained a 10%
subset still expressing CD48, CD55, CD58 and CD59. This suggests that a
somatic mutation occurring in vivo has resulted in a myeloma cell clone
with a defect for GPI-anchored membrane proteins. Cytogenetic analysis
showed shared major chromosomal abnormalities with several abnormal marker
chromosomes between the direct and cell line karyotypes. The LOPRA-2
myeloma cell line may be a suitable tool for further characterizing the
molecular nature of the GPI-anchoring defect.
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