Ultrastructural localization of a lysosomal enzyme in resin-embedded lymphocytes

Cysteine cathepsins as regulators of the cytotoxicity of NK and T cells

Cysteine cathepsins are lysosomal peptidases involved at different levels in the processes of the innate and adaptive immune responses. Some, such as cathepsins B, L, and H are expressed constitutively in most immune cells. In cells of innate immunity they play a role in cell adhesion and phagocytosis. Other cysteine cathepsins are expressed more specifically. Cathepsin X promotes dendritic cell maturation, adhesion of macrophages, and migration of T cells. Cathepsin S is implicated in major histocompatibility complex class II antigen presentation, whereas cathepsin C, expressed in cytotoxic T lymphocytes and natural killer (NK) cells, is involved in processing pro-granzymes into proteolytically active forms, which trigger cell death in their target cells. The activity of cysteine cathepsins is controlled by endogenous cystatins, cysteine protease inhibitors. Of these, cystatin F is the only cystatin that is localized in endosomal/lysosomal vesicles. After proteolytic removal of its N-terminal peptide, cystatin F becomes a potent inhibitor of cathepsin C with the potential to regulate pro-granzyme processing and cell cytotoxicity. This review is focused on the role of cysteine cathepsins and their inhibitors in the molecular mechanisms leading to the cytotoxic activity of T lymphocytes and NK cells in order to address new possibilities for regulation of their function in pathological processes.

Human immunodeficiency syndromes affecting human natural killer cell cytolytic activity

Natural killer (NK) cells are lymphocytes of the innate immune system that secrete cytokines upon activation and mediate the killing of tumor cells and virus-infected cells, especially those that escape the adaptive T cell response caused by the down regulation of MHC-I. The induction of cytotoxicity requires that NK cells contact target cells through adhesion receptors, and initiate activation signaling leading to increased adhesion and accumulation of F-actin at the NK cell cytotoxic synapse. Concurrently, lytic granules undergo minus-end directed movement and accumulate at the microtubule-organizing center through the interaction with microtubule motor proteins, followed by polarization of the lethal cargo toward the target cell. Ultimately, myosin-dependent movement of the lytic granules toward the NK cell plasma membrane through F-actin channels, along with soluble N-ethylmaleimide-sensitive factor attachment protein receptor-dependent fusion, promotes the release of the lytic granule contents into the cleft between the NK cell and target cell resulting in target cell killing. Herein, we will discuss several disease-causing mutations in primary immunodeficiency syndromes and how they impact NK cell-mediated killing by disrupting distinct steps of this tightly regulated process.

Autolysosomes accumulate during in vitro CD8+ T-lymphocyte aging and may participate in induced death sensitization of senescent cells

As autophagic inclusions accumulate in senescent fibroblasts, we wondered whether an increase in cellular fragility during in vitro lymphocyte aging may be related to an autolysosome accumulation. We established that, during long-term cultures, repeatedly stimulated T-lymphocytes acquired characteristics of replicative senescence and became progressively intolerant to activation. Cell death following stimulations: (i) corresponded to apoptosis, associated with necrosis at the end of the culture; (ii) was not, for its main part, mediated through CD95/CD178 or TNFRII/TNF alpha interactions; and (iii) occurred in spite of bcl-2 increased expression. After 14 weeks of culture, the percentage of lymphocytes containing at least one autophagic inclusion (p<0.0001) and the lipofuscin autofluorescence in lymphocytes (p<0.0001) were significantly increased. The expression of several genes regulating autophagy did not significantly vary with the age of the culture. Forty-eight hours after each stimulation, the percentage of induced cell death rose while, in the remaining living cells, the percentage of lymphocytes with autophagic vacuoles (p<0.05), with beta-galactosidase activity and the lipofuscin autofluorescence (p<0.001) significantly decreased, suggesting the preferential death of cells with autophagy. Our data support the view that the accumulation of autolysosomes in senescent lymphocytes might aggravate cellular fragility, leading to apoptosis and necrosis mainly induced by lymphocyte activation.

Secretory lysosome biogenesis in cytotoxic T lymphocytes from normal and Chediak Higashi syndrome patients

The lytic proteins mediating target cell killing are stored in the lysosomes of activated cytotoxic T lymphocytes (CTL) and are secreted upon recognition of a target cell. These secretory lysosomes cannot be detected in resting T lymphocytes. Interaction of a resting cell with a target cell activates de novo formation of secretory lysosomes. CTL clones in culture mimic this behaviour, and so provide an ideal system for studying secretory lysosome biogenesis and maturation. In the genetic disease, Chediak Higashi syndrome (CHS), all lysosomes in the cells are enlarged and reduced in number compared with wild-type (WT) cells. We have used CTL from this disease to study secretory lysosome biogenesis and maturation. We show that at early stages after activation the secretory lysosomes are identical in WT and mutant cells, and that delivery of proteins to the secretory lysosome along the biosynthetic and endocytic pathways is normal in the mutant cells. With time, the lysosomes in the mutant cells aggregate, become larger and fewer in number and eventually form giant structures. Our results show that the initial steps of secretory lysosome formation are normal in CHS, but that the organelles subsequently fuse together during cell maturation to form the giant secretory lysosomes.

Localization of lysosomal antigens in activated T-lymphocytes

The lysosomal compartment has been examined in activated T-lymphocytes by immunogold electron microscopy and subcellular fractionation. Immunoprecipitation and sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of radiolabelled extracts of the T-cells showed that they contained three antigens which are fundamental to normal lysosomal function: a representative lysosomal enzyme beta-glucuronidase, a lysosomal associated membrane protein (LAMP-1), and the cation-independent mannose 6-phosphate lysosomal enzyme targeting receptor (MPR). Immunogold labelling showed that beta-glucuronidase was present in the rough endoplasmic reticulum, the Golgi complex and Golgi-associated vesicles. The enzyme was also found to accumulate in distinct, non-Golgi organelles in which LAMP-1 was co-localized, probably lysosomes. LAMP-1 was also found in tubular elements of the Golgi and in a complex of vesicles clustered near the nucleus where MPR was also present at high density. Fractionation of homogenates from lymphocytes on Percoll gradients revealed that beta-glucuronidase was distributed throughout the low density region containing rough endoplasmic reticulum, Golgi and plasma membrane components, and the high density region which contained only lysosomal activity. Multiple immunogold electron microscopy of the latter fraction showed the presence of homogenous vesicles which had large amounts of beta-glucuronidase within the lumen, LAMP-1 at the periphery and no MPR. These vesicles were probably mature lysosomes, arising from pre-lysosomal organelles enriched for LAMP-1 and MPR.

Simultaneous measurement of cell surface and intracellular antigens by multiple flow cytometry

We have employed a method for permeabilizing lymphocytes with the detergent saponin in order to detect an intracellular protein simultaneously with surface antigens by flow cytometry (FCM). Using monoclonal antibodies specific for the murine CD2 receptor and for the lysosomal enzyme, beta-glucuronidase (Gus), we found that the expression of both of these antigens increased markedly when T cells were activated. Two sensitive methods were used to show that FCM provided an accurate measure of the actual number of CD2 and Gus molecules present in the lymphocytes. Immunogold electron microscopy revealed the precise ultrastructural localization of these different components and corroborated the specificity of the multiple labelling procedure for the simultaneous detection of surface and intracellular antigens. We also developed a three-colour FCM technique which we used to examine the changes in Gus expression in the CD4 and CD8 T cell sub-sets during activation.

The activation of resting lymphocytes is accompanied by the biogenesis of lysosomal organelles

The degradative activity of lymphocytes plays by important role in a number of essential immune functions. In the present study we have examined how the activation of resting lymphocytes, by the mitogen concanavalin A (Con A), affects three major components of the lysosomal compartment: the lysosomal enzyme beta-glucuronidase (Gus); an integral lysosomal membrane protein (LAMP-1); and the mannose 6-phosphate receptor (MPR) which directs lymphocyte enzyme transport. Resting T cells were found to contain only very low levels of these proteins, but they were actively synthesized by, and far more abundant in, stimulated lymphoblasts. Although the lysosomal antigens did not have a distinct cytoplasmic localization in the resting lymphocytes, in the activated T lymphoblasts they were present in several highly developed intracellular structures, including the rough endoplasmic reticulum and the Golgi complex. Furthermore, in these latter cells Gus was also found to be accumulated within the lumen of large vesicles which we characterized as lysosomes by the presence of LAMP-1 at the periphery and by the absence of MPR. Subcellular fractionation confirmed that these organelles were present in the activated lymphocytes only, and not in the resting T cells. Our results demonstrate that lymphocyte activation is accompanied by the synthesis of the enzymic and structural components of the lysosomal compartment which are sorted and assembled into distinct organelles in the activated cell.

Adhesion of lymphoid cells to fibroblasts in tissue culture

In this study we have examined the cellular and molecular specificity of lymphocyte interaction with fibroblasts. Using mitogen-activated T-cells, we found that attachment to fibroblasts was highly sensitive to protease treatment, and to an antibody raised against the purified lymphocyte plasma membrane, but it was not mediated by the MEL-14 surface antigen or phosphomannosyl receptors. Lymphocyte interaction with fibroblasts was also unaffected by monoclonal antibodies against the LFA-1, Mac-1, and Class II MHC antigen complexes. In contrast, adhesion of both T- and B-lymphocytes was strongly inhibited by fucoidan, a polymer of sulphated fucose, whereas fucose, mannan, and mannose 6-phosphate had no effect. Both B- and T-lymphoid cell lines were able to recognise and adhere to fibroblasts, although the marked differences between the attachment of the different types of cell did not appear to be related to their immunological function. The attachment of most of the cell lines was prevented by the presence of fucoidan, whereas the inhibition of binding of each of the lymphoid lines in the presence of the anti-T-lymphocyte plasma membrane antibody varied widely. These findings suggest that lymphocyte attachment to fibroblasts involves multiple cell surface receptors, and that these are expressed at different levels on specific T- and B-cells.

A simple technique for in situ embedding of monolayer cultures in Lowicryl K4M

A number of difficulties are encountered in embedding monolayer cultures at low temperature in Lowicryl K4M resin. In this paper we present a simple procedure using Lab-Tek Flaskettes in which fixation, processing and embedding of monolayer cultures can readily be achieved.