The calcium-binding protein calreticulin is a major constituent of lytic granules in cytolytic T lymphocytes

Cell-mediated cytotoxic mechanisms

There are two competing, but probably really complementary, models for the mechanism of cell-mediated cytotoxicity. One depends upon contact-mediated transmembrane signaling, and the other on the exocytosis of toxic materials by the killer cell. There is exciting news on both fronts. Transmembrane signaling has been shown to involve the surface molecule Fas/APO-1 on targets and its ligand on cytotoxic T cells. The Fas ligand has been cloned, and is a member of the tumor necrosis factor family. The major cytolytic molecule in the exocytosis pathway is perforin; perforin knock-out mice have been produced, and they display many intriguing abnormalities. It has been a bumper year for cytotoxicologists.

Calreticulin: from Ca2+ binding to control of gene expression

Calreticulin is a highly conserved Ca(2+)-binding/storage protein of the endoplasmic reticulum (ER). Recently, it has been shown to play a role in the control of gene expression by interacting with the DNA-binding domain of various steroid receptors. How does this ER protein gain access to the nuclear steroid receptors? We propose that calreticulin undergoes unique intracellular trafficking that allows it to colocalize with and bind to steroid receptors.

The role of apoptotic and necrotic processes in cytolysis mediated by LAK cells with different phenotypes

The role of necrotic and apoptotic pathways in cytolysis mediated by LAK cells was studied. The contribution of necrotic and apoptotic processes to cytolysis depends both on the LAK cells' phenotype and the type of target cells. CD16+/CD8+/CD3- LAK cells induced necrosis of K562 and L929 target cells. The cell death induced by CD3+/CD8+/CD16- LAK cells was found to include features of apoptotic and necrotic processes.

A calreticulin-like protein co-purifies with a '60 kD' component of Ro/SSA, but is not recognized by antibodies in Sjögren's syndrome sera

In this study, we used human tonsils for the isolation of the 60 kD component of the Ro/SSA autoantigen, following the method described by Wu et al. (J Immunol Methods 1989; 121:219-24). Western blot analyses were carried out using Ro/SSA-reactive human Sjögren's syndrome sera, to follow the autoantigen through the purification procedure. A 60 kD Ro/SSA component was eluted as a broad peak from a Mono Q column. Within this peak, a much more abundant protein, co-migrating with the Ro/SSA component on SDS-PAGE, was also eluted. The more abundant protein was further purified on a Superose 12 column and its N-terminal sequence was shown to be identical to that of human calreticulin. The 60 kD Ro/SSA autoantigen was also further purified on the Superose 12 column and was eluted as an asymmetric peak, with the majority being eluted at a position corresponding to 60 kD, whereas the calreticulin-like protein was eluted from the same column as an apparent dimer of approximately 120 kD. A panel of five Ro/SSA-reactive human sera reacted with the purified Ro/SSA antigen, but not with the calreticulin-like protein. Therefore, it is clear that the calreticulin-like protein is not a Ro autoantigen and is distinct from the 60 kD Ro/SSA antigen. As the calreticulin-like protein is a much more abundant protein than the 60 kD Ro/SSA component, its co-purification with the autoantigen on ion-exchange and its close migration with the autoantigen on SDS-PAGE may explain why peptide sequences for human calreticulin were derived from apparent 60 kD Ro/SSA antigen preparations.

Calcium uptake during mitogenic stimulation of human lymphocytes: characterization of intracellular calcium compartments and demonstration of the presence of immunoreactive calrecticulin

Phytohemagglutinin (PHA) stimulation of human peripheral blood lymphocytes (HPBL) rapidly increases 45Ca2+ uptake into intracellular pools. Detectable increase in 45Ca2+ uptake occurred only on exposure to mitogenic lectins but not with non-mitogenic lectins. However, intracellular free Ca2+ concentration [(Ca2+)i] increased comparably on exposure to either mitogenic or non-mitogenic lectins. Permeabilization of 45Ca2+ loaded cells revealed distinct pools of Ca2+ uptake. The highly digitonin sensitive pool #I (permeabilized by 0.02% digitonin) exchanged slowly and included a part that represented endoplasmic reticulum. Pool II was defined by lower digitonin sensitivity, had a much faster initial uptake. Pool III was digitonin-resistant and predominantly non-vesicular. During the first 120 min of PHA stimulation, significant increase in 45Ca2+ uptake occurred only into pool II. Progressive increase in uptake into pool I then occurred so that by 24 hours, this pool constituted the major fraction of PHA induced increment in total 45Ca2+ uptake. Using specific antibody to the calcium binding protein calreticulin, an analogous immunoreactive protein was detectable in resting HPBL. PHA stimulation led to a striking increase in abundance of immunoreactive calreticulin so that 24 hrs after PHA stimulation, there was a 28 and 3.4 fold increase in the amount of immunoreactive calreticulin present in the non-particulate fraction and the total particulate membrane fraction, respectively. A major part (72%) of the total cellular immunoreactive calreticulin in PHA stimulated cells at 24 hrs was released into the medium after permeabilization of lymphocytes with 0.02% digitonin, corresponding to the location of calcium uptake pool I.

Cell-mediated cytotoxicity: contact and secreted factors

The list of cells with cytotoxic potential now may include small resting T cells, but the exact nature of 'lethal hit delivery' by cytotoxic T lymphocytes remains elusive. Cell-mediated cytotoxicity by cytotoxic T lymphocytes is a complex, multistep process which seems likely to be mediated by several different pathways. Recent experimental evidence for the functioning of a novel cytotoxic mechanism through a target cell's surface receptor illustrates and emphasizes the necessity to study the interactions of cytotoxic T lymphocytes and target cells as a whole. Progress is evident in the description of molecular requirements for triggering cytotoxicity, cell-cell contacts and the regulation of the effector responses of cytotoxic T lymphocytes by extracellular, intracellular and granular proteins. Extracellular Ca(2+)-dependent secretion of perforin and protease(s) may explain several aspects of cellular cytotoxicity, whereas the apoptosis-mediating cell surface Fas protein is now implicated in Ca(2+)-independent cytotoxicity.