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Thema: 3 x 3 (3 Slides in 3 Minutes) Abstract Präsentationen - I
49.14
Decrease of sialic acid residues as an eat-me signal on apoptotic and viable lymphoblasts
Meesmann H.1, Heyder P.1, Blank N.2, Lorenz H.2, Schiller M.1
(1) Universitätsklinikum Heidelberg, Medizinische Klinik V, Heidelberg, (2) Universitätsklinikum Heidelberg,
Medizinische Klinik V, Sektion Rheumatologie, Heidelberg
Zielsetzung
The silent removal of apoptotic cells is essential for cellular homeostasis in multicellular organisms, and
defects in the clearance of apoptotic cells are involved in the pathogenesis of autoimmune diseases. In vitro,
several eat-me signals have been identified as mediators of apoptotic cell recognition. Though, the distinct
mechanisms of apoptotic cell clearance are not fully deciphered to date. In the present study, we analyzed
the expression of sialic acids on the surface of apoptotic material and the role of these sugar molecules for
an effective engulfment of apoptotic debris.
Methodik
As a cellular model we used activated lymphoblasts which represent activated T-cells and are quite
susceptible to induction of apoptosis. After apoptosis induction lymphoblasts were stained by plant lectins to
detect sialic acid residues on their surface. The used lectins MAL I (Maackia amurensis lectin I) and SNA
(Sambucus nigra agglutinin) specifically bind α 2,3- and α 2,6- linked terminal sialic acids. Detection of lectin
binding was performed by flow cytometry and confocal microscopy. Phagcytosis of apoptotic cells was
quantified by a flow cytometric phagocytosis assay and by confocal microscopy. For analysis of phagocytosis
we always used an autologous cell system, in which lymphoblasts were co-cultured together with monocytederived phagocytes.
Ergebnisse
After apoptosis induction by UV-B irradiation, we observed a significant decrease of sialic acid expression on
the surface of apoptotic cells. Thus, we have been interested whether this decrease of sialic acids might
represent an eat-me signal for professional phagocytes. To investigate this, cleavage of sialic acids was
induced by the addition of neuraminidase to apoptotic cells and apoptotic bodies. Addition of this enzyme
resulted in a dose dependent decrease of sialic acids on the cellular surface. Further, the engulfment of
neuraminidase treated apoptotic cells/bodies by monocyte-derived phagocytes was increased. Interestingly,
we also observed an increased phagocytosis, even if viable lymphoblasts were treated by neuraminidase. In
this context, it is important to note that neuraminidase did not induce any apoptotic or necrotic cell death by
itself. These results are all the more surprising, as phagocytosis of viable lymphocytes is usually not to be
observed in an autologous cell system. Analyzing cytokines in the supernatant of phagocytes, co-cultured
with neuraminidase-treated lymphoblasts and apoptotic bodies, we observed an increased secretion of the
pro-inflammatory cytokine TNF-alpha.
Schlussfolgerung
Our findings suggest that a decrease of terminal sialic acids represents an eat-me signal for the
phagocytosis of apoptotic material. Interestingly, this signal seems to be independent from other features of
apoptotic cell death (e.g. phosphatidylserine exposure) as neuraminidase treatment did even cause
phagocytosis of viable lymphoblasts in an autologous cell system.