Differential expression of MHC class II antigens and cathepsin L by subtypes of cortical epithelial cells in the rat thymus: an immunoelectron microscopic study

CD74: an emerging opportunity as a therapeutic target in cancer and autoimmune disease

INTRODUCTION

CD74, also known as the invariant chain, participates in several key processes of the immune system, including antigen presentation, B-cell differentiation and inflammatory signaling. Despite being described more than 3 decades ago, new functions and novel interactions for this evolutionarily conserved molecule are still being unraveled. As a participant in several immunological processes and an indicator of disease in some conditions, it has potential as a therapeutic target.

AREAS COVERED

The relationship between the structure of CD74 variants and their physiological functions is detailed in this review. The function of CD74 in several cell lineages is examined with a focus on the interactions with cathepsins and, in an inflammatory milieu, the pro-inflammatory cytokine macrophage migratory inhibitory factor. The role of CD74 signaling in inflammatory and carcinogenic processes is outlined as is the use of CD74 as a therapeutic target (in cancer) and tool (as a vaccine).

EXPERT OPINION

CD74 has several roles within the cell and throughout the immune system. Most prominent amongst these are the complex relationships with MIF and cathepsins. Modulation of CD74 function shows promise for the effective amelioration of disease.

Colocalization of cystatin M/E and cathepsin V in lamellar granules and corneodesmosomes suggests a functional role in epidermal differentiation

Cystatin M/E is a cysteine protease inhibitor with two distinct binding sites for papain-like cysteine proteases (family C1) and the asparaginyl endopeptidase (AEP) legumain of family C13. We have previously demonstrated that deficiency of cystatin M/E in mice causes ichthyosiform skin changes and barrier disruption, which could be caused by unrestrained AEP activity. Recently, we provided biochemical evidence that human cathepsin V (CTSV) and cathepsin L (CTSL) are additional biological targets for human cystatin M/E. To address the possible role of these three proteases and their inhibitor in epidermal differentiation, we investigated the localization of these proteins in normal human skin. Whereas CTSL and AEP were broadly expressed in epithelial cells of the skin, we found a specific colocalization of cystatin M/E and CTSV in the stratum granulosum and in the root sheets of the hair follicle, using immunofluorescence microscopy. Immunoelectron microscopy revealed that cystatin M/E and CTSV are separately transported within the lamellar granules. Cystatin M/E was also found in the extracellular space in the stratum corneum associated with corneodesmosomes, where it was closely associated with CTSV. Based on the striking stratum-specific colocalization of cystatin M/E and CTSV, we propose that these molecules could have an important role in epidermal differentiation and desquamation.

Ultrastructural alterations of the cortical epithelial cells of the irradiated and recovering rat thymus

To understand the roles of cortical thymic epithelial cells (CTECs) in T-lymphocyte development, we analyzed rat thymi recovering from irradiation (6Gy), at the ultrastructural level. The morphological alterations in the CTECs were most prominent during the third to fifth day of recovery, when proliferating thymocytes were observed in the vicinity of the CTECs. The most striking finding among the alterations in the CTECs after irradiation was a cytoplasmic vacuolization with an increased amount of granular and membranous content. The granular content was observed as loosely aggregated structures or finely dispersed granules and dense bodies. The membranous content appeared in various forms including vesicular, tubular, and irregular membranous structures and myelin figures. The above features are characteristic of the hyperfunctional state of CTECs with increased secretion activities, which suggests their important roles in the repopulation and maturation of the cortical thymocytes during recovery after irradiation.

Morphological and immunohistochemical changes to thymic epithelial cells in the irradiated and recovering rat thymus

The present study analyzed morphological and immunohistochemical changes of thymic epithelial cells in the irradiated and recovering rat thymus. Observations showed the number of thymocytes was initially severely reduced after irradiation but abruptly increased on days 3 to 5 after 6 Gy and on days 7 to 11 after 8 Gy irradiation. To analyse the mechanisms for this abrupt recovery of the thymocytes after irradiation, the expression of p63 in the normal and irradiated thymus was immunohistochemically studied as the expression of this antigen may be related to the proliferation of epithelial cells. In the fetal thymus tissue, thymic epithelial cells were the principal cell type that stained strongly positive for p63. The sporadic expression of p63 was also observed in the normal adult thymus tissue, especially in the subcapsular region. An increased number of p63- positive cells in the thymus after irradiation indicates that repair or renewal of the thymic epithelial cells may be taking place because p63 is more specific to transient amplifying thymic epithelial cells. A RT-PCR analysis of p63 expression in irradiated and regenerating thymus tissue also showed an increased expression of p63 after irradiation compared with that of the normal thymus. These results suggest that changes in the thymic microenvironment-especially in relation to the repair and renewal of thymic epithelial cells- may have an important influence on thymocyte proliferation in the normal thymus as well as in the irradiated and recovering thymus.

Thymus cell-cell interactions

The recent advances in molecular biology and genetics, as well as the progress of in vitro techniques, have provided a more coherent image of the thymic function on the molecular level. But they have shifted the attention away from studies on the cellular level, which are necessary to clarify the biological roles of different cell types of the thymic microenvironment. The structure and function of the normal thymus depend on mutual interactions between thymocytes and nonlymphocyte cells. In this review a detailed description of morphological and phenotypic features of both maturing thymocytes and nonlymphocyte cells is given. The recent genetic and biochemical data are presented in conjunction with cytological results to enlighten the thymus cell-cell interactions during thymopoiesis and organization of thymic microstructure. Special emphasis is put on the experimental approaches, which may be used to study the interactions between thymocytes and nonlymphocyte cells in vivo.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

Identification and characterization of major histocompatibility complex class II compartments in cortical thymic epithelial cells

Previous studies have concentrated on elucidating the subcellular localization of major histocompatibility (MHC) class II molecules mainly in B cells, macrophages, and dendritic cells. Despite very rich cell-surface expression of MHC class II molecules by cortical thymic epithelial cells (cTECs), little is known regarding the expression of these molecules by cTECs at the subcellular level. In the present study we focused on the identification and characterization of MHC class II compartments (MIICs) in cTECs in situ by immunogold electron microscopy (IEM). We found that MHC class II molecules were located exclusively in the cytoplasmic vacuoles, and we identified these MHC class II molecule-containing cytoplasmic vacuoles as MIICs in cTECs. These MIICs were immunopositive for early endosomal, late endosomal, and lysosomal markers. Moreover, in these MIICs, MHC class II molecules were colocalized with cathepsin L, H2-DM, class II-associated invariant chain (Ii), and class II-associated invariant chain peptide (CLIP). Similarly, Ii molecules were colocalized with endosomal and lysosomal markers, cathepsin L, and H2-DM in the vacuoles. Taken together, these results suggest that MIICs in cTECs represent conventional endocytic compartments. The colocalization of MHC class II molecule or Ii with cathepsin L and H2-DM in the MIICs suggests that MIICs in cTECs may be sites of Ii degradation and peptide loading.

Spatial relation between major histocompatibility complex-restricted antigen receptor-bearing thymocytes and subtypes of thymic epithelial cells

R73, a monoclonal antibody that recognizes a rat T cell surface antigen of TCR(alphabeta), was used to identify thymocytes that express major histocompatibility complex-restricted antigen receptors, and to define the spatial relation between these receptor-bearing thymocytes and individual thymic cortical and medullary epithelial subtypes by ultrastructural immunohistochemistry. We show that in both the cortex and medulla 1) the thymocytes that reacted with R73 antibodies exhibited three staining patterns: cytoplasmic-only staining, simultaneous cytoplasmic and surface membrane staining, and surface membrane-only staining; 2) the subcapsular/perivascular epithelial cells (ECs) were usually associated with thymocytes expressing perinuclear staining only; and 3) the surface membrane areas of thymocytes that expressed antigen receptors made contact with pale and intermediate ECs, but not with dark cells. These results suggest that thymic selection of major histocompatibility complex restriction and/or tolerance may occur by interaction of the receptors on maturing thymocytes with major histocompatibility complex antigens on thymic ECs in general, and on pale and intermediate subtypes in particular.