The MRC OX-44 antigen marks a functionally relevant subset among rat thymocytes

A monoclonal antibody to a constant determinant of the rat T cell antigen receptor that induces T cell activation. Differential reactivity with subsets of immature and mature T lymphocytes

mAb R73 detects a T cell-specific surface molecule consisting of two disulfide-linked subunits of 40 and 46 kD, respectively, on 97% of peripheral rat T cells, as defined by the OX-52 marker. Of the few OX-52+ R73- cells, none are CD4+ but many express the CD8 antigen known to be present on rat NK cells. mAb R73 is mitogenic for unseparated spleen cells and for purified T cells. In the absence of non-T "accessory cells", stimulation by R73 requires artificial crosslinking of the mAb and is largely dependent on exogenous IL-2. Overnight incubation of purified T cells with crosslinked R73 mAb induces blastoid transformation, IL-2-R expression, and modulation of the R73 antigen. In the rat thymus, mature medullary cells express the R73 determinant at the same level as peripheral T cells, whereas 94% of CD4-CD8- thymocytes are R73-. The major CD4+8+ thymocyte population contains 25% R73- and 70% R73low cells. Thymocytes of the CD4-CD8+OX-44- subpopulation that are the direct precursors of CD4+CD8+ cells display a continuum of R73 antigen density from undetectable to very low levels. We conclude that R73 is most likely directed at a constant determinant of the rat alpha/beta heterodimeric TCR and suggest that CD8+ immature thymocytes are the first cells in the T cell differentiation pathway to express this molecule at their surface.

Cross-linking of the T cell antigen receptor interferes with the generation of CD4+8+ thymocytes from their immediate CD4-8+ precursors

The majority of thymocytes are immature cells co-expressing the surface markers CD4 and CD8. About two thirds of these cells also express the T cell antigen receptor (TcR), though at a level distinctly lower than found on mature T cells. The direct precursors of these "double-positive" thymocytes are cycling cortical blast cells of the CD4-8+ phenotype. Using a new monoclonal antibody to a constant determinant of the rat TcR alpha/beta, it is shown here that (a) about 50% of these CD8 "single-positive" committed precursor cells already express the TcR alpha/beta, though at very low levels, (b) during short-term suspension culture in medium supplemented only with fetal calf serum they not only acquire CD4 but also TcR alpha/beta levels characteristic of CD4,8 "double-positive" thymocytes, and (c) cross-linking of the TcR during culture inhibits the acquisition of the CD4 antigen in the majority of these cells.

Macrophages phagocytose thymic lymphocytes with productively rearranged T cell receptor alpha and beta genes

The thymus gland is important for the formation of competent T lymphocytes. However, there is long-standing evidence that greater than 95% of newly formed thymocytes do not emigrate to peripheral lymphoid tissues but instead die locally. We have identified a rapid and selective pathway for thymocyte turnover in vitro. The mechanism entails binding, uptake, and digestion by macrophages. The susceptible cells are a subpopulation of double-positive thymocytes. These thymocytes can be enriched by virtue of their high buoyant density in Percoll and prove to have low levels of surface CD3 and little or no surface TCR. However TCR-alpha and -beta genes have undergone rearrangement, and full length alpha and beta transcripts are abundant. Therefore many double-positive cells rearrange and express TCR genes but do not have normal levels of TCR on the cell surface. We propose that thymocytes that undergo high turnover in situ are unable to form receptors that can be selected by MHC molecules in the thymus, and that these cells are recognized and cleared by the macrophage.

Adoptively transferred experimental allergic encephalomyelitis in chimeric rats: identification of transferred cells in the lesions of the central nervous system

Experimental allergic encephalomyelitis (EAE) was induced by adoptive transfer of myelin basic protein (MBP)-activated LEW spleen cells into (LEW x PVG/c) F1----LEW chimeras. By double-immunofluorescent staining using OX27, which is specific for RT1c, and monoclonal antibodies (mAb) against various T-cell antigens (TAg), inflammatory cells in the lesions of the central nervous system (CNS) were categorized into MBP-activated and transferred LEW T cells (TAg+ OX27-), accompanying T cells (TAg+ OX27+) of chimera origin and non-T cells (TAg- OX27+). Examination of the lesions at various stages of EAE revealed that transferred OX19 (CD5)+ T cells accounted for 46% of the total number of inflammatory cells at the preclinical stage, became reduced to 23% at the clinical stage and recovered to a level between those of the preclinical and clinical stages at the recovery stage. In parenchymal infiltrates, 93% of the total T cells were transferred cells at the preclinical stage, whereas 66% were present in perivascular aggregates. At the clinical stage, the proportion of transferred T cells in the parenchyma was not different from that in the perivascular cuffs. At the recovery stage, the proportion of transferred T cells in the parenchyma was increased. Collectively, MBP-activated and transferred T cells first appeared in the CNS parenchyma followed by infiltration of T and non-T cells of recipient (chimera) origin. All these inflammatory cells formed the lesions of full-blown EAE. At the recovery stage, inflammatory cells decreased in number in all the compartments of the CNS. Transferred T cells formed the major proportion of parenchymal infiltrates at this stage. These findings strongly suggest that transferred T cells remain in the CNS parenchyma longer than cells of chimera origin and that antigen-activated T cells have well-expressed CNS-homing activity.

Significance of thymus differential ratio in immunologic status of graft tolerance

LEW with BDE-heart graft received 0 (control), 15, or 40 mg cyclosporine (CsA)/kg b. wt. per day. On postoperative days 3, 5, 7, 10, and 14 in four animals each weight and cell count of thymus and spleen were determined, and thymus and spleen cell subpopulations were examined with monoclonal antibodies. The same tests were performed in FiS heart graft recipients without immunosuppression and ungrafted LEW which received 15 or 40 mg CsA. We expressed alterations in thymocyte subpopulations by using the differentiation ratio (DR), i.e., differentiated in % of all T-cells and by the ratio of helper to suppressor/cytotoxic T-cells (Th-Ts/c). In graft rejection the thymus showed no significant change in DR or Th-Ts/c. However, in the CsA-induced graft tolerance DR was elevated and at the same time Th-Ts/c declined, both showing maximum values on days 5 and 7 and a return to normal thereafter. FiS graft recipients exhibited similar thymus alterations as tolerant recipients, but less marked. In CsA-treated ungrafted LEW, elevation of DR was slight after 15 mg but very strong after 40 mg CsA (93% on day 7), and it did not return to normal in the latter group. Th-Ts/c was decreased in these ungrafted animals, but not as strongly as in tolerant graft recipients. Such thymus alterations were not observed in graft rejection. Spleen weights were strongly increased in graft rejection and unchanged in graft tolerance. Splenic Ts/c and Th-Ts/s were increased in CsA-treated tolerant recipients but not in graft rejection. We conclude that elevation of DR and decline of thymic Th-Ts/c in the initial postoperative phase are indicators of graft tolerance in organ recipients.

Influence of cyclosporin A on growth of an acute T-cell leukaemia in PVG rats

Our objective was to examine the effect of cyclosporin A (CsA; 25 or 12.5 mg/kg) on growth of an acute (Roser) T-cell leukaemia in male PVG rats. The leukaemic blasts were shown (by immunocytochemical analysis) to have a mature, T-helper-cell phenotype, i.e., OX-19 (CD5) +/- , W3/25 (CD4)+, OX44+, MHC-class I+, OX-26+, corresponding to a population comprising 5% of normal rat medullary thymocytes. Animals received 20 X 10(3) viable tumour cells intramuscularly (day 0) and were given either CsA (25 or 12.5 mg/kg) or drug vehicle by gavage from day 0 or day 14, by which latter time leukaemic blasts normally appeared in the circulation. Administration of the higher dose of CsA from day 0 or day 14 significantly delayed the appearance of leukaemic cells in the peripheral circulation, whereas treatment with 12.5 mg/kg was without significant effect. CsA whole blood levels on day 17 were twice as high in leukaemic rats as in normal controls. Leukaemic infiltration of the spleen and the liver was reduced on day 17 after 25 mg/kg CsA, but no such effect was observed in lymph nodes or kidneys. A heterogeneous, host "reactive" cell population, which developed in response to the leukaemia, was inhibited by CsA, indicating that the effect of the drug was probably not mediated by host defence mechanisms. In CsA-treated leukaemic animals, there was biochemical evidence of synergistic impairment of glomerular and tubular function.

A CD3- subset of CD4-8+ thymocytes: a rapidly cycling intermediate in the generation of CD4+8+ cells

T lymphocytes with the surface phenotype CD4+8- and CD4-8+ are considered to be representative of functionally mature cells. We show here that adult murine thymus contains a subpopulation of CD4-8+ cells that differ from CD4-8+ cells found in the periphery in that they do not express the T cell receptor-associated CD3 molecular complex. Such CD3-4-8+ thymocytes are cortisone sensitive and rapidly cycling in situ. Furthermore, in contrast to mature T cells, most CD3-4-8+ thymocytes express low levels of CD5 and high levels of the B2A2 antigen. CD3-4-8+ thymocytes fail to respond to a variety of mitogenic stimuli in vitro but do give rise upon short-term culture to CD4+8+ cells. It is suggested that CD3-4-8+ thymocytes represent a transitional stage of thymus differentiation between the CD4-8- and CD4+8+ compartments.

Analysis of T cell receptor beta chains in rat thymus, and rat C alpha and C beta sequences

In development, T cells first express their alpha beta antigen receptors in the thymus, where they may undergo selection processes leading to major histocompatibility complex (MHC) restriction and tolerance. A high proportion of thymocytes are thought to fail this selection in some way and to be destined for intrathymic death. These cells are categorized as the "cortical type" since they constitute most of the cortical cells; they express both CD4 and CD8 antigens but only very low levels of MHC class I antigens. One suggested cause of thymocyte death is a failure to produce a functional alpha beta T cell receptor (Tcr) due to errors in the rearrangements of germline DNA, resulting in V regions being absent or incorrectly spliced to the other segments of the transcribed gene. We have sequenced from the C region through to the V region of 14 rat Tcr beta chain clones isolated from thymocyte cDNA libraries. Of the 14, 13 have complete and correct rearrangements, whereas one was expressed from an unrearranged gene. Most of these clones are likely to be derived from the cortical population, for Northern blot analysis showed that these cells and total thymocytes expressed similar amounts of beta chain mRNA. Furthermore, the RNA from cortical-type cells contained a very similar ratio of full-length to truncated beta chain mRNA as did activated thymocytes and mature T lymphocytes. The data imply that defective beta chain gene rearrangement is not a major cause of failure in the selection of thymocytes. The sequences of the rat Tcr alpha and beta chain constant regions are also reported.

Absence of donor-type major histocompatibility complex class I antigen-bearing microglia in the rat central nervous system of radiation bone marrow chimeras

Localization of bone marrow-originated cells in the central nervous system (CNS) of the rat was investigated by using bone marrow chimeras. In order to do this, Lewis rats which carry major histocompatibility complex (MHC) class I antigens haplotype 1 (RT1.Al) were reconstituted with (Lew X PVG)F1 (RT1.Al/c) bone marrow cells after lethal irradiation. Transferred bone marrow cells were detected by immunohistochemical staining using a monoclonal antibody, OX27, specific for haplotype c of rat MHC class I antigens (RT1.Ac). The spleen and thymus of chimeric rats were fully reconstituted with transferred F1 cells 4 weeks after bone marrow transplantation. At this stage, mononuclear cells in the subarachnoid space of the CNS expressed OX27 antigen indicating that they were of bone marrow origin. A few OX27-positive blood cells were scattered in the CNS parenchyma 4-12 weeks after reconstitution. Ramified microglia, however, remained OX27-negative. Bone marrow-derived microglia were not observed throughout the period of examination until 24 weeks. In addition, experimental allergic encephalomyelitis (EAE) was induced in chimeric rats in order to augment the expression of MHC class I antigens on microglia. Even under this condition, no OX27-positive microglia were observed. Taken together, ramified microglia might be of neuroectodermal origin and there is little possibility that the microglia are derived from the bone marrow. However, if the ramified microglia are derived from blood cells, the microglia may be expected to have characteristic cell kinetics from the following points: (1) the precursor cells of the microglia may enter the CNS only at the perinatal stage; and (2) even under the condition in which lymphocytes and macrophages enter the CNS as observed in EAE, the precursor cells of the microglia are not supplied from the blood.

Unfractionated human thymocytes have a lower proliferative capacity than CD3-4-8- ones but have a similar capacity for expression of interleukin 2 receptors and production of interleukin 2

CD3-4-8- and unfractionated thymocytes were compared for their capacity to proliferate, to express interleukin 2 (IL-2) receptor, and to secret IL-2. Phorbol ester and Ca2+ ionophore were used as mitogens. CD3-4-8- thymocytes responded vigorously when stimulated with phorbol ester in the presence of IL-2 or in combination with Ca2+ ionophore. In contrast, unfractionated thymocytes responded weakly when stimulated with either of these mitogens. Surprisingly, however, the stimulation of these populations with either phorbol ester plus IL-2 or phorbol ester plus ionophore induced a high and similar level of IL-2 receptor expression in both thymocyte populations. A similar level of IL-2 secretion in both populations was also obtained when they were stimulated with a combination of phorbol ester plus ionophore. These results suggest that during the maturation process, the majority of thymocytes lose their capacity to be activated by some mitogens, although they maintain their capacity to secrete IL-2 and to express the IL-2 receptor.

An intermediate cell in thymocyte differentiation that expresses CD8 but not CD4 antigen

Thymocytes of CD4-,CD8+,OX-44- phenotype have been shown to be an intermediate of thymopoiesis that give rise to cells of CD4+, CD8+, OX-44- normal cortical thymocyte phenotype both in vitro and in vivo during thymic regeneration.

Similarities in sequences and cellular expression between rat CD2 and CD4 antigens

The MRC OX-34 antigen of rat T lymphocytes was purified and peptide sequences were obtained. Oligonucleotide probes were synthesized and cDNA clones coding for the antigen were isolated and sequenced to yield a predicted protein sequence for the molecule that fitted the peptide data. Comparison of this sequence with that for human CD2 determined by Sewell et al. showed that OX-34 is rat CD2. The primary structure of the molecule was notable for a moderately large cytoplasmic domain of unusual sequence and also for its highly significant relationship to CD4 antigen in the membrane proximal extracellular region and the transmembrane sequence. A relationship to the Ig superfamily can be argued for the two extra cellular domains of CD2, even though neither fits the standard pattern for Ig-related domains. Within the T lymphocyte lineage, rat CD2 seemed to be present on all stages with the exception of approximately 50% of the thymic CD4-,CD8- cells. In addition, the antigen was prominent on most macrophages in the spleen but not found on peritoneal or liver macrophages. CD4 antigen is also expressed on T lymphocytes and macrophages, and thus CD2 and CD4 appear similar in their cellular expression as well as structural characteristics.