The immune response to secondary necrotic cells

Abstract

When apoptotic cells are not cleared in an efficient and timely manner, they progress to secondary necrosis and lose their membrane integrity. This results in a leakage of immunostimulatory, danger associated molecular patterns (DAMPs), similar to accidental (or primary) necrosis. However, primary necrosis is a sudden event with an inadvertent release of almost unmodified DAMPs. Secondary necrotic cells, in contrast, have gone through various modifications during the process of apoptosis. Recent research revealed that the molecules released from the cytoplasm or exposed on the cell surface differ between primary necrosis, secondary necrosis, and regulated necrosis such as necroptosis. This review gives an overview of these differences and focusses their effects on the immune response. The implications to human physiology and diseases are manifold and will be discussed in the context of cancer, neurodegenerative disorders and autoimmunity.

Keywords: Apoptosis; Cancer immunotherapy; Efferocytosis; Inflammation; Primary necrosis; Secondary necrosis.

Fig. 1

Transmission electron microscopic images of viable, primary necrotic, early apoptotic and secondary necrotic cells. Human Jurkat cells were cultured and cell death (primary necrosis and apoptosis) was induced as previously described [26]. Apoptotic cells and secondary necrotic cells were separated from each other by fluorescence-activated cell sorting. Then cells were fixed in 2.5% glutaraldehyde in cacodylate buffer prior to immersion in 1% OsO₄ solution, and dehydration in a series of ethanol. The dehydrated samples were infiltrated gradually in mixtures of propylene oxide and epoxy resin Agar 100. Thin section (60–80 nm) were cut with an ultramicrotome, mounted on copper grids, counterstained with uranyl acetate and lead citrate and examined at 120 kV in a ZEISS Libra 120 electron microscope. a Viable cell with a normal morphology including intact cell membrane (white arrow) and nuclear membrane (black arrow). b Primary necrotic showing the loss of membrane integrity (white arrow) and low cytoplasm density (black arrow). A high DNA content can still be observed (white arrowhead). c Apoptotic cell with marked by chromatin condensation and karyorrhexis (black arrows) and intact plasma membrane (white arrow). d Secondary necrosis showing a disintegrated cell membrane (black arrows) and loss of chromatin. Black bar 2 µm

Fig. 2

Immunomodulatory signals of secondary necrotic cells. Schematic presentation of a secondary necrotic cell and two apoptotic microparticles. The plasma membrane is permeable (symbolized by a broken membrane) and phosphatidylserine (PS) is exposed on its surface (indicated as red membrane sections). PS presentation leads to binding of different proteins (MFG-E8, Gas6, proteins S, C1q, and annexin A1) which are all recognized by antigen presenting cells. They stimulate a clearance of the secondary necrotic cell but inhibit at the same time an induction of inflammation. The intracellular ATP has been consumed during early apoptosis resulting in a lower ATP release from secondary necrotic cells than from primary necrotic cells. Intracellular HMGB1 migrates to the nucleus and binds to nucleosomes, which have been separated from each other during early apoptosis (DNA laddering). The extracellular proteins FSAP, DNase I and C1q enter the cell and bind to HMGB1-nucleosome complexes. This leads to a DNA degradation and release of HMGB1-nucleosome complexes. It is not clear whether FSAP, DNase I and C remain bound to the released complexes. Urate, which accumulates as degradation product of DNA, forms MSU microcrystals and is then released. Both, HMGB1-nucleosome complexes and MSU microcrystals are pro-inflammatory signals. For further details see Table 1 and text. HMGB1 high mobility group protein B1, FSAP factor VII-activating protease, C1q complement component C1q, MFG-E8 milk fat globule-EGF factor 8, Gas6 growth arrest-specific 6, MSU mono sodium urate, PS phosphatidylserine. (Color figure online)

Fig. 3

Conceptual model of the immune response to primary necrosis, apoptosis and secondary necrosis. The succession of cell death, immune response and regeneration of the damaged tissue is symbolized in circles: one for the sequence of events after induction of primary necrosis and one for apoptosis. The latter circle is usually shortened a by timely clearance of apoptotic cells without inflammatory attraction of macrophages and neutrophils. If apoptotic cells proceed to secondary necrosis they induce an inflammatory response which can become chronic and even induce an adaptive immune response (see text for further details)