Trogocytosis in multiple myeloma
- Publication Type:
- Thesis
- Issue Date:
- 2012
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The term trogocytosis is used to describe the fast, cell-to-cell contact-dependent
transfer of membrane proteins between cells, with T and B-lymphocytes, natural killer
(NK) cells, antigen presenting cells (APC) and tumour cells being the most widely
studied. This study aimed to: (1) identify the extent of trogocytosis in patients with
multiple myeloma (MM), a malignancy of bone marrow plasma cells, compared with
the other B cell malignancies, (2) to identify some of the molecules involved with
trogocytosis in these patients and then (3) determine if cells that had acquired
molecules had altered function.
An in vitro model of trogocytosis was established in which plasma cell lines and flowsorted bone marrow plasma cells (CD38++) of patients with MM (n=11) or the
malignant B cells from patients with chronic lymphocytic leukaemia (CLL)
(CD5+CD19+) (n=4) and Waldenstrom macroglobulinaemia (WM) (CD19+) were
biotinylated and then cultured with either patient or normal mononuclear cells. The
acquisition of biotinylated membrane proteins was determined by flow cytometry and
confocal microscopy. Screening for potential molecules involved suggested that
CD86 and HLA-G were likely candidates for trogocytosis. CD86 is a co-stimulatory
molecule at the immune synapse and HLA-G is a non-classical MHC class I molecule
which prevents antigen-specific cytolysis by cytotoxic T lymphocytes (CTLs),
inhibits the function of circulating NK cells and prevents proliferation of allogeneic
CD4+ T cells. These observations have lead to the hypothesis that expression of HLAG
may aid in the escape of tumours from immune surveillance.
T cells acquired significantly more biotinylated proteins (mean=13.55%) than B cells
(mean=2.43%; t=2.80; p<0.05) or NK cells (mean=3.15%; t=2.57; p<0.05). There was
no significant difference between levels of biotin transferred to T cells from either
plasma cell lines or primary plasma cells and acquisition was the same with
autologous and allogeneic T cells. Significantly more trogocytosis was observed in
myeloma patients than other B cell malignancies (n=5) as <1% T cells acquired
membrane fragments when cultured with malignant B cells from patients with CLL or
WM (t =3.86; p<0.05). Upon culture with biotinylated CD3+ flow-sorted normal T cells, approximately 2% of CD38++ plasma cells acquired membrane fragments,
suggesting that in patients with myeloma, trogocytosis was predominantly
unidirectional.
Although HLA-G expression was found on 0.02 – 0.56% of normal T cells (mean
=0.23%), 20% of MM patients (11 of 56) demonstrated a level of HLA-G+ CD3+
T cells above the normal range. Addition of flow-sorted CD3+ HLA-Gpos T cells led to a
reduction in the proliferation of carboxyfluorescein succinimidyl ester (CFSE)-
labelled CD3+ HLA-Gneg T cells stimulated with anti-CD3/CD28 beads and this
inhibition was greater than the inhibition due to CD38++ HLA-Gpos plasma cells
(t=2.64; p=0.046). The CD3+ HLA-Gpos T cells acquired inhibitory function but were
not natural T regulatory cells as they were CD25neg. Overall survival was significantly
worse for the 11/46 patients with HLA-Gpos plasma cells ( 2=12.4; p<0.0004).
Flow cytometric analysis of CD38++ bone marrow plasma cells from MM patients
showed varied HLA-G expression ranging from 0.2% to 96% (n=46). The clinical
relevance of HLA-Gpos plasma cells was demonstrated by a significant reduction in
overall survival (n= 46; χ2= 12.4; p<0.004). CD86 expression on T cells of myeloma
patients (n=98) ranged from 0 – 30% (normals = 0 – 2.7%; n=10). T cells from
myeloma patients (n=7), when co-cultured with CD86 expressing plasma cells, were
found to acquire significantly (p<0.0001) higher levels of CD86.
This study reports several new findings. It has shown that trogocytosis is more
common in multiple myeloma than other B cell malignancies, is primarily
unidirectional, HLA independent and T cells are more likely to be involved than other
lymphocytes. T cells which acquire tumour antigens may have altered function and it
has been demonstrated that HLA-Gpos T cells form a new subset of acquired
regulatory T cells that inhibit the proliferation of HLA-Gneg T cells and therefore
protect MM cells against the host’s immune defences.
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