The differentiation pathway of T lymphocytes. Evidence for two differentiated cell types

Quantitative ultrastructure of cytolytic lymphocytes mediating allograft rejection in the mouse. I. Cellular alterations in T lymphocytes during specific target cell lysis

A quantitative ultrastructural analysis of cytolytic T lymphocytes (CTL) is presented which allows both the distinction of these cells from normal T lymphocytes and permits the demonstration of ultrastructural alterations of putative CTL following interaction with target cells (TC). Alloreactive CTL were generated in C57BL/10 mice receiving intraperitoneal fibroblastic allografts and target-binding splenic lymphocytes (TBSL) were concentrated by specific immunoadsorption on fibroblast monolayers. TBSL were subjected to ultrastructural quantification either at the onset of TC interaction or following 30 or 60 min incubation at 37 degrees C. By means of simple stereological relationships it was shown that, in comparison with normal, non-cytolytic splenic T lymphocytes, TBSL were slightly larger cells, displaying around 60% more cytoplasm, a similarly-sized nucleus and approximately triple the volume of Golgi apparatus. During the first 30 min of interaction with TC, the target binding surface of the TBSL plasma membrane decreased in area. This change was accompanied by a polarization of the TBSL towards the target. Incubation of lymphocytes with TC for a further 30 min resulted in a general polarization of lymphocytic cellular constituents away from the TC. These results were only attainable by objective quantitative analysis and are discussed in relation to possible mechanisms of CTL-mediated lysis.

Distribution and morphological characteristics of the pit cells in the liver of the rat

Pit cells, on which almost no further contributions have been presented since the first report by Wisse et al. (1976), are described in detail in the rat liver. These cells show several characteristic features: 1) "rod-cored vesicles", a new type of vesicular inclusion observed first in our study; 2) electron-dense granules, which we consider to arise from multivesicular bodies by the accumulation of dense material; and 3) well-developed pseudopodia. Although these features clearly differentiate pit cells from conventional lymphocytes, these two cell types display similarities (i) in a number of ultrastructural features, (ii) in the pattern of their intralobular distribution, and (iii) in their presence in the spleen and peripheral blood.

The graft-versus-host reaction in the rabbit spleen. Ultrastructural evidence for two differentiated T-cell lines

Lethally irradiated (900 rads), bone-marrow-protected rabbits were given allogeneic thymocytes intravenously. Autoradiography by means of local splenic infusion of (H3) thymidine revealed progeny of two differentiated T-cell lines. Electron microscopy confirmed these observations in detail. An analogy with cellular immunity reactions in an "isolated T-cell system", performed in draining lymph nodes after a variety of antigenic stimuli is presented. The functional properties of these T-blasts and their lymphoid descendants are discussed.

Blood lymphocyte ultrastructure and deoxyribonucleic acid content in children with systemic lupus erythematosis

Two varieties of peripheral blood lymphocytes have been disclosed in systemic lupus erythematosus (SLE) cases: one showing signs of degradation and nuclear chromatine elimination and the other one manifesting a state of biological activation, possibly of an immunologic nature. This karyostructural lymphocyte heterogeneity in SLE may cause a great scattering of these cells on histograms in respect to their nuclear deoxyribonucleic acid content determined by cytophotometry. On the other hand, the expressiveness of the scattering and the degree of predominance of negative tendency towards proliferation (with a shift to the left from 2 n) may thereby serve as a very objective quantitative indication of nuclear structure degradation and of loss by lymphocytes of chromatine with deoxyribonucleic acid during SLE.

Microcinematographic and electron microscopic analysis of target cell lysis induced by cytotoxic T lymphocytes

A study was carried out to determine the sequence of events of T-cell mediated target cell lysis in microcinematography and electron microscopy. Highly efficient cytotoxic T lymphocytes (CTL) were generated in vivo and in vitro using preimmunized spleen cells and purification procedures. Such CTL were highly specific. This specificity correlated well with the number of adhesions formed between CTL and targets and this criterion was used to study killer-target cell interaction. Microcinematography showed that target cell lysis at the single cell level, despite time variations, could be clearly separated into three phases: (a) a recognition phase, visible by random crawling of CTL over the target cell surface until firm contact was established; (b) a post-recognition phase, during which firm contact between CTL and target was maintained without gross modification of either cell; (c) a phase of target cell disintegration, mainly characterized by vigorous blebbing of the cell membrane resulting in a motionless carcass of the target cell but not in its total dissolution. Only later this carcass decayed and formed a necrotic ghost. Electron microscopic observations were put into sequence according to microcinematography. Post-recognition phase was characterized by a tight apposition of the membranes of CTL and target cell. No gap junctions could be observed. During target cell disintegration, profound cytoplasmic and nuclear changes occurred simultaneous with surface blebbing. Most noticeable were extensive internal vacuolization, mitochondrial swelling, nuclear pycnosis and dissolution of the nucleolus. These observations suggested that target cell lysis does not start with a surface phenomenon similar to complement lysis, but a process involving practically the whole cell simultaneously. It is conceivable, therefore, that the signal from the CTL is transmitted across the target cell, and that the switch to sudden cell death is manipulated deep inside the cell.

Electron microscopy of cellular immunity reactions in B-cell deprived rabbits. Thymus derived antigen reactive cells, their micro-environment and progeny in the lymph node

Three times sublethal total body X-irradiation with thymus shielding--at 14 days' intervals--established an 'isolated T-cell system'. Cellular immunity reactions in draining lymph nodes of rabbits have been analyzed at the ultrastructural level after challenge with horse spleen ferritin, the chemical sensitizer, 2-phenyl-4-ethoxymethylene-5-oxazolone and after skin allografting. A basic pattern of interdigitating cells (IDC) is described as a specific constituent of the thymus-dependent areas of peripheral lymphoid tissue. Their ultrastructural features and presumable functional relationship to the afferent loop of cellular immunity reactions are presented. The differentiation pathway of two T-cell lines is shown and discussed in relation to cellular and thymus-dependent humoral immunity reactions.

Macrophage-lymphocyte cluster formation in the medullary sinus of lymph node after immunization with sheep red blood cells (SRBC)

Normally the lymphatic sinuses of the lymph node are loosely packed with lymphocytes and free macrophages as well as with macrophages adhering to the fibrocellular trabeculae. After immunization with SRBC cluster formation occurs in the medullary sinuses of rats between a central macrophage and peripherally located lymphocytes. These rosette-like clusters are nearly identical with the clusters found during primary and secondary immune response against SRBC in vitro and seem to be the in vivo equivalent for the same immune response.

Lymphocyte-epithelium interaction during rejection of nonisogeneic rat tracheal grafts

Rat tracheal tissues were maintained in organ culture for 1 to 2 weeks and then implanted heterotopically into syngeneic and outbred hosts. The grafts and surrounding tissues were removed at regular intervals and examined by light and electron microscopy. Allogeneic grafts were consistently rejected, and a constant pattern of immunologic response was observed during rejection. Infiltration by mononuclear cells occurred initially in the subepithelial region. Subsequently, small lymphocytes migrated through the basal lamina and insinuated themselves between the differentiated epithelial cells. Larger cells resembling activated lymphocytes were also present in the intraepithelial infiltrate. Both type of lymphocytes had cellular processes in contact with epithelial cells, but there was no associated death of epithelial cells. Later, the infiltrate in some areas of the epithelium inclded dividing cells and other large cells exhibiting ultrastructural features suggesting blast transformation. Epithelial cell death was evident during this phase. The sequence of events may reflect a process of recognition and specific cytotoxic action by lymphocytes.

Experimental models for prevention of graft-versus-host reaction in bone marrow transfusion. I. Selective suppression and augmentation of splenomegaly and cytotoxicity

Induction and suppression of splenomegaly and cytotoxicity against C57BL/L cells were studied in (AKR X C57BL/6) F1 hybrid adult mice after the transfer of AKR lymphoid and bone marrow cells. 1) Splenomegaly and cytotoxicity were dissociated in the developmental stages of the graft-versus-host reaction. When lymphoid and bone marrow cells of normal AKR mice were injected into F1 recipients, splenomegaly was prominent on days 5 and 7, but cytotoxicity of spleen cells was not detected. Splenomegaly became less prominent but the cytotoxicity became detectable on day 14 after the injection. 2) Cytotoxic activity of spleen cells of F1 recipients was suppressed by the treatment of AKR donors with C57BL/6 lymphoid cells in Freund's complete adjuvant. Splenomegaly, however, was substantially enhanced by such a treatment of the donors. On the other hand, induction of the cytotoxic activity was facilitated by the treatment of donors with C57BL/6 skin grafts. 3) F1 hybrid mice could be protected from the graft-versus-host reaction by the injection of AKR anti-C57BL/6 serum or pretreatment of AKR donors with sonicated cellular antigens of C57BL/6.

Electron Microscopic Observations on T and B Lymphocytes from Spleens of Syngeneic Radiation Chimaeras

The ultrastructure of T and B lymphocytes has been examined in long-term syngeneic chimaeras and age-control mice. Spleen cell suspensions from these mice were passed through glass wool columns to obtain pure lymphocyte populations. These cells were then separated into T and B lymphocytes by nylon wool columns, and their purity was tested by cytotoxicity assays with anti-ϑ serum. Electron microscopic observations on such separated T and B lymphocytes did not reveal morphological differences except when the cells were fully differentiated, either as mature (T2) cells or plasmacells. In particular, T2 cells showed a very high cytoplasmic density, attributable to the presence of a larger number of microfilaments with respect to immature (T1) cells. In long-term chimaeras a significantly larger number of T2 cells was found as compared to age-control mice, and this morphological observation is correlated with the differences in immune reactivity and leukemia incidence previously described in these mice.

The differentiation of cytotoxic T lymphocytes in vitro: an ultrastructural study

T killer cells were identified morphologically in two different culture systems. The first system allowed identification by comparing under identical culture conditions two types of stimulation, one leading to cytotoxic activity levels that were very high (H-2 stimulation) and the other to levels which were practically nil (M-locus stimulation). A second system allowed identification by the use of a collaborative system in which relatively pure precursors of T killer cells were mixed with precursors of T amplifier cells from which they were distinguishable by their Thy-l antigen. In this latter case, surface labelling techniques had to be used. As in the in vivo situation, T killer cells were medium-sized pale cells with evidence of a well-developed secretory and motile apparatus. No evidence of highly active protein synthesis was found. Other T cells were dark and mainly derived from the T amplifier population, again consistent with in vivo findings. There was considerable background proliferation, even in the absence of any added antigen, and this included the formation of some well-developed plasma cells.

A study of proteolysis as a possible mechanism for T-cell-mediated target cell lysis

The hypothesis that proteolysis is implicated in T-cell-mediated target cell lysis was tested in two ways: first, various inhibitors of protease activity were used either during or before the cytotoxicity test. Ovomucoid, TPCK, and PMSF proved to be not or only marginally inhibitory, whereas the considerable inhibitory action of TLCK was shown to most likely be a toxic effect of the inhibitor. Second, radioactive substrates for cathepsin activity were used, either free in the medium or fixed on the surface of the target cells, to detect a possible breakdown of these substrates by a release of intracellular cathepsins during the cytotoxicity test. Very little dialysable breakdown products were formed, and there was no difference between their amount in either a homologus or a heterologous system. It is concluded that proteolysis probably is not the mechanism by which target cells are killed. Alternative mechanisms are discussed.

Morphological definition of thymocyte subpopulations

An attempt was made to distinguish immature from mature, immunocompetent thymocytes in the adult mouse using morphological criteria. All procedures enriching for competent thymocytes also enrich small cells whose morphology is very similar to the small thymocytes that have been described in the medulla of the thymus (Abe and Ito, 1970). A heterogeneity of competent thymocytes is likely since the enriched populations always contain a number of medium sized cells. These latter cells show spontaneous DNA-synthetic activity and are probably the source of thymus proliferative activity. Owing to their characteristic morphology these two cell types might represent the precursors for each of the two differentiated T cell types that have been found to occur in delayed type hypersensitivity (Matter, 1974).