Do Corticosteroid Injections for the Treatment of Pain Influence the Efficacy of Adenovirus Vector-Based COVID-19 Vaccines?

Myth: Corticosteroid injection for the treatment of pain is known to decrease the efficacy of the adenovirus vector-based vaccines for COVID-19.

Fact: There is currently no direct evidence to suggest that a corticosteroid injection before or after the administration of an adenovirus vector-based COVID-19 vaccine decreases the efficacy of the vaccine.

•However, based on the known timeline of hypothalamic-pituitary-adrenal (HPA) axis suppression following epidural and intraarticular corticosteroid injections, and the timeline of the reported peak efficacy of the Janssen and AstraZeneca vaccines, physicians should consider timing an elective corticosteroid injection such that it is administered no less than two weeks prior to and no less than two weeks following a COVID-19 adenovirus vector-based vaccine dose, whenever possible.

•We emphasize the importance of risk/benefit analysis and shared decision-making in determining the timing of corticosteroid injections for pain indications in relation to receipt of a COVID-19 vaccine given that patient-specific factors will vary.

Do Corticosteroid Injections for the Treatment of Pain Influence the Efficacy of mRNA COVID-19 Vaccines?

MYTH

Corticosteroid injection for the treatment of pain and inflammation is known to decrease the efficacy of the messenger ribonucleic acid (mRNA) vaccines for coronavirus disease 2019 (COVID-19).

FACT

There is currently no direct evidence to suggest that a corticosteroid injection before or after the administration of an mRNA COVID-19 vaccine decreases the efficacy of the vaccine.However, based on the known timeline of hypothalamic-pituitary-adrenal (HPA) axis suppression following epidural and intraarticular corticosteroid injections, and the timeline of the reported peak efficacy of the Pfizer-BioNTech and Moderna vaccines, physicians should consider timing an elective corticosteroid injection such that it is administered no less than 2 weeks prior to a COVID-19 mRNA vaccine dose and no less than 1 week following a COVID-19 mRNA vaccine dose, whenever possible.

Myeloid-derived suppressor cell and regulatory T cell frequencies in canine myasthenia gravis: A pilot study

Myasthenia gravis (MG) is a T cell-dependent, B cell-mediated autoimmune disease. Little is known about its cellular pathogenesis in dogs. This study provides the first preliminary assessment of the frequency of myeloid-derived suppressor cells (MDSCs) and regulatory T cells (Tregs) in the peripheral blood of dogs with seropositive generalized MG. No alteration in frequency of either MDSCs or Tregs in dogs with MG was observed when compared to those in either seronegative dogs with diagnoses other than MG, or healthy dogs. A longitudinal study in three dogs with MG revealed no correlation between the relative numbers of either population and the clinical course of disease. Neither the frequency of MDSCs nor of Tregs showed a correlation with anti-AChR antibody titer in dogs with MG. These findings suggest that aberrations in the frequency of either immunosuppressive population do not occur in MG, but they need to be validated in large-scale prospective studies.

Bacterial Killing Activity of Polymorphonuclear Myeloid-Derived Suppressor Cells Isolated From Tumor-Bearing Dogs

Polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs) are implicated in the progression and outcome of a variety of pathological states, from cancer to infection. Our previous work has identified three antimicrobial peptides differentially expressed by PMN-MDSCs compared to conventional neutrophils isolated from dogs, mice, and human patients with cancer. We therefore hypothesized that PMN-MDSCs in dogs with cancer possess antimicrobial activity. In the current work, we observed that exposure of PMN-MDSCs to Gram-negative bacteria (Escherichia coli) increased the expression of reactive oxygen species by the PMN-MDSCs, indicating that they are capable of initiating an anti-microbial response. Electron microscopy revealed that the PMN-MDSCs phagocytosed Gram-negative and Gram-positive (Staphylococcus aureus) bacterial species. Lysis of bacteria within some of the PMN-MDSCs suggested bactericidal activity, which was confirmed by the recovery of significantly lower numbers of bacteria of both species following exposure to PMN-MDSCs isolated from tumor-bearing dogs. Our data therefore indicate that PMN-MDSCs isolated from dogs with cancer, in common with PMNs, have phagocytic and bactericidal activity. This nexus of immunosuppressive and antimicrobial activity reveals a hitherto unrecognized function of MDSCs.

Identification of a Multipotent Progenitor Population in the Spleen That Is Regulated by NR4A1

The developmental fate of hematopoietic stem and progenitor cells is influenced by their physiological context. Although most hematopoietic stem and progenitor cells are found in the bone marrow of the adult, some are found in other tissues, including the spleen. The extent to which the fate of stem cells is determined by the tissue in which they reside is not clear. In this study, we identify a new progenitor population, which is enriched in the mouse spleen, defined by cKit⁺CD71^lowCD24^high expression. This previously uncharacterized population generates exclusively myeloid lineage cells, including erythrocytes, platelets, monocytes, and neutrophils. These multipotent progenitors of the spleen (MPPS) develop from MPP2, a myeloid-biased subset of hematopoietic progenitors. We find that NR4A1, a transcription factor expressed by myeloid-biased long term-hematopoietic stem cells, guides the lineage specification of MPPS. In vitro, NR4A1 expression regulates the potential of MPPS to differentiate into erythroid cells. MPPS that express NR4A1 differentiate into a variety of myeloid lineages, whereas those that do not express NR4A1 primarily develop into erythroid cells. Similarly, in vivo, after adoptive transfer, Nr4a1-deficient MPPS contribute more to erythrocyte and platelet populations than do wild-type MPPS. Finally, unmanipulated Nr4a1^-/- mice harbor significantly higher numbers of erythroid progenitors in the spleen compared with wild-type mice. Together, our data show that NR4A1 expression by MPPS limits erythropoiesis and megakaryopoeisis, permitting development to other myeloid lineages. This effect is specific to the spleen, revealing a unique molecular pathway that regulates myeloid bias in an extramedullary niche.

Notch signaling directs splenic resident hematopoietic stem cell fate decisions

Adult hematopoiesis occurs primarily in the bone marrow (BM), generating mature blood cells as well as maintaining a heterogeneous pool of self-renewing hematopoietic stem cells (HSCs). However, studies suggest that the BM may not be the only site of adult hematopoiesis: during times of BM hematopoietic distress stemming from radiation or disease, the spleen can independently reconstitute the blood. Given that the spleen does indeed contain a small, resident population of lineage-sca1+ckit+CD48-CD150+ long-term (LT)-HSCs with potent reconstitution ability, we investigated the peripheral blood cell contribution of these extra-medullary HSCs by transplanting UBC-GFP Tg reporter splenic fragments into a splenectomized syngenic wildtype mouse and tracking the blood cell production over time. While the spleen gives rise to both mature myeloid and lymphoid cells, we find that the proportion is skewed in favor of T cells. We see spleen-derived donor HSCs generating CD4+CD8+ double positive thymocytes for over five months post-transplant, suggesting that this peripheral T cell population derives from a long-lived stem or progenitor cell resident in the spleen. RNA sequencing demonstrates that splenic LT-HSCs are more responsive to Notch signaling than BM LT-HSCS, as indicated by increased levels of Hes1 transcription and decreased levels of the granulocyte/monocyte (GM) lineage priming genes Mpo, Gfi1, and Fcgr2b. Taken together with our finding that splenic HSCs have higher levels of cytoplasmic Notch2, we suggest that the Notch2-Hes1 axis is simultaneously repressing GM priming and enforcing T cell programming even among the most immature HSCs resident in the spleen.

The role of the orphan nuclear receptor NR4A1 in erythro-myelopoiesis

Studies of hematopoietic stem cells (HSCs) have advanced our understanding of the intracellular signals and microenvironmental interactions that influence HSC fate. Our lab is interested in the role of orphan nuclear receptor NR4A1, an immediate response gene sensitive to external stimuli, in HSC development. NR4A1 regulates the development of specific, mature hematopoietic cell lineages from both the innate and adaptive immune system including patrolling monocytes, a mature myeloid cell subset. More recently, we have shown that NR4A1 expression also identifies a subpopulation of myeloid-biased long-term HSCs in the bone marrow. Given that NR4A1 directs both immature and differentiated cell types, we investigated its role in myeloid cell maturation. Within the bone marrow and spleen progenitor cell compartments, we find that NR4A1 is exclusively expressed by myeloid progenitors and not by more restricted megakaryocyte, erythroid, or common monocyte progenitors. Nr4a1−/−mice exhibit skewed myeloid progenitor cell populations, exhibiting a 2-fold increase in CD105+CD150− erythroid progenitors and cKit+Ter119+CD71+ pro-erythroblasts in the spleen but not in the bone marrow. In vitro cultures of Nr4a1−/−splenic myeloid progenitors also show that NR4A1 restricts the frequency of erythroid cells, yet bone marrow transplant studies show similar erythroid progenitor reconstitution from both WT and Nr4a1−/− donors. Taken together, our data reveal a new role for NR4A1 as both a direct and indirect modulator of erythropoiesis.

The orphan nuclear receptor NR4A1 specifies a distinct subpopulation of quiescent myeloid-biased long-term HSCs

Hematopoiesis is maintained throughout life by self-renewing hematopoietic stem cells (HSCs) that differentiate to produce both myeloid and lymphoid cells. The NR4A family of orphan nuclear receptors, which regulates cell fate in many tissues, appears to play a key role in HSC proliferation and differentiation. Using a NR4A1(GFP) BAC transgenic reporter mouse we have investigated NR4A1 expression and its regulation in early hematopoiesis. We show that NR4A1 is most highly expressed in a subset of Lin(-) Sca-1(+) c-Kit(+) CD48(-) CD150(+) long-term (LT) HSCs, and its expression is tightly associated with HSC quiescence. We also show that NR4A1 expression in HSCs is induced by PGE2, a known enhancer of stem cell engraftment potential. Finally, we find that both NR4A1(GFP+) and NR4A1(GFP-) HSCs successfully engraft primary and secondary irradiated hosts; however, NR4A1(GFP+) HSCs are distinctly myeloid-biased. These results show that NR4A1 expression identifies a highly quiescent and distinct population of myeloid-biased LT-HSCs.

NR4A1 (Nur77) deletion polarizes macrophages toward an inflammatory phenotype and increases atherosclerosis

RATIONALE

NR4A1 (Nur77) is a nuclear receptor that is expressed in macrophages and within atherosclerotic lesions, yet its function in atherosclerosis is unknown.

OBJECTIVE

Nur77 regulates the development of monocytes, particularly patrolling Ly6C(-) monocytes that may be involved in resolution of inflammation. We sought to determine how absence of nuclear receptor subfamily 4, group A, member 1 (NR4A1) in hematopoietic cells affected atherosclerosis development.

METHODS AND RESULTS

Nur77(-/-) chimeric mice on a Ldlr(-/-) background showed a 3-fold increase in atherosclerosis development when fed a Western diet for 20 weeks, despite having a drastic reduction in Ly6C(-) patrolling monocytes. In a second model, mice deficient in both Nur77 and ApoE (ApoE(-/-)Nur77(-/-)) also showed increased atherosclerosis after 11 weeks of Western diet. Atherosclerosis was associated with a significant change in macrophage polarization toward a proinflammatory phenotype, with high expression of tumor necrosis factor-α and nitric oxide and low expression of Arginase-I. Moreover, we found increased expression of toll-like receptor 4 mRNA and protein in Nur77(-/-) macrophages as well as increased phosphorylation of the p65 subunit of NFκB. Inhibition of NFκB activity blocked excess activation of Nur77(-/-) macrophages.

CONCLUSIONS

We conclude that the absence of Nur77 in monocytes and macrophages results in enhanced toll-like receptor signaling and polarization of macrophages toward a proinflammatory M1 phenotype. Despite having fewer monocytes, Nur77(-/-) mice developed significant atherosclerosis when fed a Western diet. These studies indicate that Nur77 is a novel target for modulating the inflammatory phenotype of monocytes and macrophages and may be important for regulation of atherogenesis.

The transcription factor NR4A1 (Nur77) controls bone marrow differentiation and the survival of Ly6C- monocytes

The transcription factors that regulate differentiation into the monocyte subset in bone marrow have not yet been identified. Here we found that the orphan nuclear receptor NR4A1 controlled the differentiation of Ly6C- monocytes. Ly6C- monocytes, which function in a surveillance role in circulation, were absent from Nr4a1-/- mice. Normal numbers of myeloid progenitor cells were present in Nr4a1-/- mice, which indicated that the defect occurred during later stages of monocyte development. The defect was cell intrinsic, as wild-type mice that received bone marrow from Nr4a1-/- mice developed fewer patrolling monocytes than did recipients of wild-type bone marrow. The Ly6C- monocytes remaining in the bone marrow of Nr4a1-/- mice were arrested in S phase of the cell cycle and underwent apoptosis. Thus, NR4A1 functions as a master regulator of the differentiation and survival of 'patrolling' Ly6C- monocytes.

Immature CD4+CD8+ thymocytes and mature T cells regulate Nur77 distinctly in response to TCR stimulation

The orphan steroid receptor, Nur77, is thought to be a central participant in events leading to TCR-mediated clonal deletion of immature thymocytes. Interestingly, although both immature and mature murine T cell populations rapidly up-regulate Nur77 after TCR stimulation, immature CD4+CD8+ thymocytes respond by undergoing apoptosis, whereas their mature descendants respond by dividing. To understand these developmental differences in susceptibility to the proapoptotic potential of Nur77, we compared its regulation and compartmentalization and show that mature, but not immature, T cells hyperphosphorylate Nur77 in response to TCR signals. Nur77 resides in the nucleus of immature CD4+CD8+ thymocytes throughout the course of its expression and is not found in either the organellar or cytoplasmic fractions. However, hyperphosphorylation of Nur77 in mature T cells, which is mediated by both the MAPK and PI3K/Akt pathways, shifts its localization from the nucleus to the cytoplasm. The failure of immature CD4+CD8+ thymocytes to hyperphosphorylate Nur77 in response to TCR stimulation may be due in part to decreased Akt activity at this developmental stage.

Notch activation is an early and critical event during T-Cell leukemogenesis in Ikaros-deficient mice

The Ikaros transcription factor is both a key regulator of lymphocyte differentiation and a tumor suppressor in T lymphocytes. Mice carrying a hypomorphic mutation (Ik(L/L)) in the Ikaros gene all develop thymic lymphomas. Ik(L/L) tumors always exhibit strong activation of the Notch pathway, which is required for tumor cell proliferation in vitro. Notch activation occurs early in tumorigenesis and may precede transformation, as ectopic expression of the Notch targets Hes-1 and Deltex-1 is detected in thymocytes from young Ik(L/L) mice with no overt signs of transformation. Notch activation is further amplified by secondary mutations that lead to C-terminal truncations of Notch 1. Strikingly, restoration of Ikaros activity in tumor cells leads to a rapid and specific downregulation of Notch target gene expression and proliferation arrest. Furthermore, Ikaros binds to the Notch-responsive element in the Hes-1 promoter and represses Notch-dependent transcription from this promoter. Thus, Ikaros-mediated repression of Notch target gene expression may play a critical role in defining the tumor suppressor function of this factor.

Cutting edge: identification of the targets of clonal deletion in an unmanipulated thymus

Autoreactive thymocytes can be eliminated by clonal deletion during their development in the thymus. The precise developmental stage(s) at which clonal deletion occurs in a normal thymus has been difficult to assess, in large part because of the absence of a specific marker for TCR-mediated apoptosis. In this report, we reveal that Nur77 expression can be used as a specific marker of clonal deletion in an unmanipulated thymus and directly identify TCRintCD4+CD8+ and semimature CD4+CD8- thymocytes as the principal targets of deletion. These data indicate that clonal deletion normally occurs at a relatively late stage of development, as cells mature from CD4+CD8+ thymocytes to single-positive T cells.

Deciphering the role of Notch signaling in lymphopoiesis

Components of the Notch signaling pathway are expressed during multiple stages of lymphoid development. Consistent with its function during invertebrate development, Notch signaling is proposed to have a central role in lymphoid cell-fate specification. Recent studies show that Notch signaling is a proximal event in T-cell commitment from a common lymphoid progenitor. The role of Notch at later stages of lymphoid development is controversial, but recent data suggest models that may help clarify observations. Current studies suggest that Notch activity is cell-context dependent and interactions between Notch and other environmental receptors are integrated during cell-fate decisions. Furthermore, the requirement for precise regulation of Notch activity is evident from human and murine neoplasms in which dysregulated Notch signaling leads to T-cell leukemia. Future studies that identify the stages of lymphoid development where Notch signaling is physiologically active and the exact targets of Notch signaling that are relevant to lymphopoiesis should significantly improve our understanding of Notch function in T- and B-cell development.

An invitation to T and more: notch signaling in lymphopoiesis

Cell fate decisions in metazoans are regulated by Notch signals. During lymphoid development, Notch influences a series of cell fate decisions involving multipotent progenitors. This review focuses on current views and lingering uncertainties about Notch function in lymphoid cells.

Differential requirement for SLP-76 domains in T cell development and function

The hematopoietic cell-specific adaptor protein, SLP-76, is critical for T cell development and mature T cell receptor (TCR) signaling; however, the structural requirements of SLP-76 for mediating thymopoiesis and mature T cell function remain largely unknown. In this study, transgenic mice were generated to examine the requirements for specific domains of SLP-76 in thymocytes and peripheral T cells in vivo. Examination of mice expressing various mutants of SLP-76 on the null background demonstrates a differential requirement for specific domains of SLP-76 in thymocytes and T cells and provides new insight into the molecular mechanisms underlying SLP-76 function.

Separation of Notch1 promoted lineage commitment and expansion/transformation in developing T cells

Notch1 signaling is required for T cell development. We have previously demonstrated that expression of a dominant active Notch1 (ICN1) transgene in hematopoietic stem cells (HSCs) leads to thymic-independent development of CD4(+)CD8(+) double-positive (DP) T cells in the bone marrow (BM). To understand the function of Notch1 in early stages of T cell development, we assessed the ability of ICN1 to induce extrathymic T lineage commitment in BM progenitors from mice that varied in their capacity to form a functional pre-T cell receptor (TCR). Whereas mice repopulated with ICN1 transduced HSCs from either recombinase deficient (Rag-2(-/)-) or Src homology 2 domain--containing leukocyte protein of 76 kD (SLP-76)(-/)- mice failed to develop DP BM cells, recipients of ICN1-transduced Rag-2(-/)- progenitors contained two novel BM cell populations indicative of pre-DP T cell development. These novel BM populations are characterized by their expression of CD3 epsilon and pre-T alpha mRNA and the surface proteins CD44 and CD25. In contrast, complementation of Rag-2(-/)- mice with a TCR beta transgene restored ICN1-induced DP development in the BM within 3 wk after BM transfer (BMT). At later time points, this population selectively and consistently gave rise to T cell leukemia. These findings demonstrate that Notch signaling directs T lineage commitment from multipotent progenitor cells; however, both expansion and leukemic transformation of this population are dependent on T cell-specific signals associated with development of DP thymocytes.

Notch1 regulates maturation of CD4+ and CD8+ thymocytes by modulating TCR signal strength

Notch signaling regulates cell fate decisions in multiple lineages. We demonstrate in this report that retroviral expression of activated Notch1 in mouse thymocytes abrogates differentiation of immature CD4+CD8+ thymocytes into both CD4 and CD8 mature single-positive T cells. The ability of Notch1 to inhibit T cell development was observed in vitro and in vivo with both normal and TCR transgenic thymocytes. Notch1-mediated developmental arrest was dose dependent and was associated with impaired thymocyte responses to TCR stimulation. Notch1 also inhibited TCR-mediated signaling in Jurkat T cells. These data indicate that constitutively active Notch1 abrogates CD4+ and CD8+ maturation by interfering with TCR signal strength and provide an explanation for the physiological regulation of Notch expression during thymocyte development.

Immature CD4+CD8+ thymocytes do not polarize lipid rafts in response to TCR-mediated signals

TCR-mediated stimulation induces activation and proliferation of mature T cells. When accompanied by signals through the costimulatory receptor CD28, TCR signals also result in the recruitment of cholesterol- and glycosphingolipid-rich membrane microdomains (lipid rafts), which are known to contain several molecules important for T cell signaling. Interestingly, immature CD4(+)CD8(+) thymocytes respond to TCR/CD28 costimulation not by proliferating, but by dying. In this study, we report that, although CD4(+)CD8(+) thymocytes polarize their actin cytoskeleton, they fail to recruit lipid rafts to the site of TCR/CD28 costimulation. We show that coupling of lipid raft mobilization to cytoskeletal reorganization can be mediated by phosphoinositide 3-kinase, and discuss the relevance of these findings to the interpretation of TCR signals by immature vs mature T cells.

Third ventricular cysts and membranes unsuspected on conventional CT and MRI

Three cases are presented of symptomatic cysts or membranes within the third ventricle interfering with CSF flow and presenting as non-communicating triventricular hydrocephalus. None was visible on conventional CT or MRI, two being discovered at neuroendoscopy and one only with a specific MRI sequence designed to show CSF partitioning.

Paediatric brain tumours: an embryological perspective

Primitive neuroectodermal tumours are amongst the most common paediatric tumours of the central nervous system. These tumours are composed of undifferentiated cells and a variable component of more differentiated cell types. Most analysis of these tumours has focused on molecules normally found in the differentiated cells or those found in all primitive neuronal precursors. In this article we describe recent advances in understanding of the molecular processes involved in normal neurogenesis. We discuss the relevance of these data to the biology of neuronal tumours and describe strategies we and others have adopted to investigate the usefulness of molecules found in undifferentiated neuronal tissues in understanding the events which underlie oncogenesis in this tumour type.

Expression of the Bcl-2 family member A1 is developmentally regulated in T cells

During T cell development, cells that fail to meet stringent selection criteria undergo programmed cell death. Thymocyte and peripheral T cell susceptibility to apoptosis is influenced by expression of Bcl-2 family members, some of which are expressed in a developmentally patterned manner. We previously showed developmentally regulated expression of A1, an anti-apoptotic Bcl-2 family member, among B cell developmental subsets. Here we show that cells of the T lineage also express A1 in a developmentally regulated manner. Both A1 mRNA and A1 protein are readily detectable in the thymus, and while present among DN cells, A1 mRNA is up-regulated to very high levels among double-positive (DP) thymocytes. It is then down-regulated to moderate levels among single-positive (SP) thymocytes, and finally expressed at approximately 25-fold lower levels among mature SP CD4(+) and CD8(+) lymph node T cells than among DP thymocytes. Furthermore, we find that in vitro TCR ligation up-regulates A1 expression among both DP and SP thymocytes. Together, these data show that A1 expression is developmentally regulated in T lymphocytes and is responsive to TCR signaling, suggesting that A1 may play a role in maintaining the viability of DP thymocytes.

Receptor avidity and costimulation specify the intracellular Ca2+ signaling pattern in CD4(+)CD8(+) thymocytes

Thymocyte maturation is governed by antigen-T cell receptor (TCR) affinity and the extent of TCR aggregation. Signals provided by coactivating molecules such as CD4 and CD28 also influence the fate of immature thymocytes. The mechanism by which differences in antigen-TCR avidity encode unique maturational responses of lymphocytes and the influence of coactivating molecules on these signaling processes is not fully understood. To better understand the role of a key second messenger, calcium, in governing thymocyte maturation, we measured the intracellular free calcium concentration ([Ca2+]i) response to changes in TCR avidity and costimulation. We found that TCR stimulation initiates either amplitude- or frequency-encoded [Ca2+]i changes depending on (a) the maturation state of stimulated thymocytes, (b) the avidity of TCR interactions, and (c) the participation of specific coactivating molecules. Calcium signaling within immature but not mature thymocytes could be modulated by the avidity of CD3/CD4 engagement. Low avidity interactions induced biphasic calcium responses, whereas high avidity engagement initiated oscillatory calcium changes. Notably, CD28 participation converted the calcium response to low avidity receptor engagement from a biphasic to oscillatory pattern. These data suggest that calcium plays a central role in encoding the nature of the TCR signal received by thymocytes and, consequently, a role in thymic selection.

Positive selection as a developmental progression initiated by alpha beta TCR signals that fix TCR specificity prior to lineage commitment

During positive selection, immature thymocytes commit to either the CD4+ or CD8+ T cell lineage ("commitment") and convert from short-lived thymocytes into long-lived T cells ("rescue"). By formal precursor-progeny analysis, we now identify what is likely to be the initial positive selection step signaled by alpha beta TCR, which we have termed "induction". During induction, RAG mRNA expression is downregulated, but lineage commitment does not occur. Rather, lineage commitment (which depends upon the MHC class specificity of the alpha beta TCR) only occurs after downregulation of RAG expression and the consequent fixation of alpha beta TCR specificity. We propose that positive selection can be viewed as a sequence of increasingly selective developmental steps (induction-->commitment-->rescue) that are signaled by alpha beta TCR engagements of intrathymic ligands.

Altered functional responsiveness of thymocyte subsets from CD3delta-deficient mice to TCR-CD3 engagement

CD3delta-deficient (delta degrees) mice are defective in alphabeta T cell development. Here we explore the capacity of TCR-CD3 signaling complexes expressed on delta degrees thymocytes to mediate the following functional outcomes in response to antibody cross-linking: (i) the transition from the CD4-CD8- to CD4+CD8+ stage, (ii) the transition from the CD4+CD8+ to CD4+CD8- or CD4-CD8+ stages and (iii) the induction of apoptosis. We provide evidence that CD3deltaepsilon complexes are dispensable for mediating the anti-CD3-mediated CD4-CD8- to CD4+CD8+ transition. On the other hand, CD3delta is critical at the CD4+CD8+ stage. We demonstrate that CD4+CD8+ thymocytes from delta degrees mice, unlike delta degrees CD4-CD8- thymocytes and wild-type CD4+CD8+ thymocytes, require prolonged or consecutive stimuli to elicit functional responses. Depending on the nature of the secondary stimulus, delta degrees thymocytes can be induced to undergo apoptosis or preferential maturation to the CD4-CD8+ stage. Taken together these results indicate that the signaling capacity of the TCR-CD3 complex is noticeably altered in the absence of CD3delta. The essential role of CD3delta at the CD4+CD8+ stage of development correlates with the onset of TCRalpha rearrangement, consistent with a critical structural and/or functional relationship between CD3delta and TCRalpha.

Thymic function in young/old chimeras: substantial thymic T cell regenerative capacity despite irreversible age-associated thymic involution

Age-associated thymic involution results in a diminished capacity to regenerate T cell populations, although the magnitude of this effect is unknown. In this report, thymic function was studied in aged vs. young adult mice after lethal irradiation and administration of T cell-depleted bone marrow (BM) from young mice. Abnormalities observed in aged thymi (reduced thymocyte numbers, histologic abnormalities) were not reversed by administration of young BM via bone marrow transplantation (BMT), but aged thymi displayed a normal thymocyte subset distribution and appropriately deleted MIs-reactive T cells after BMT. Aged BMT recipients regenerated significantly reduced numbers of splenic T cells compared to young recipients and showed increased peripheral expansion of thymic emigrants since a higher proportion of BM-derived T cells expressed a memory phenotype in aged vs. young BMT recipients. Because peripheral expansion of thymic emigrants could substantially increase the number of thymic progeny present in the spleen, we sought to measure thymic T cell regenerative capacity after BMT in a setting devoid of peripheral expansion. To do this, TCR-transgenic (Tg+) T cell-depleted BM was administered to aged and young recipients lacking antigen specific for the Tg+ TCR. Aged recipients regenerated approximately 50 % of the TCR Tg+ cells regenerated in young BMT recipients, providing evidence that even very aged thymi retain the capacity to regenerate significant numbers of mature T cell progeny. Therefore, thymic function is reduced with aged but it is not lost, suggesting that therapeutic approaches to enhance thymic function may be successful even in very aged hosts.

T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus

Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4+CD8+ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

Developmentally regulated expression of peanut agglutinin (PNA)-specific glycans on murine thymocytes

Intrathymic maturation of T lymphocytes is characterized by variable expression of O-linked Gal beta 1,3GalNAc glycans reactive with peanut agglutinin (PNA) lectin. Recent studies on human thymocytes show that conversion from PNA+ to PNA- phenotype is correlated with increased expression of alpha 2,3 O-linked sialyltransferase (ST), which sialylates Gal beta 1,3GalNAc glycans, masking their binding sites for PNA. Interestingly, alpha 2,3 O-linked ST expression is highest within the regions of the thymus containing the most immature and most mature thymocyte subsets, suggesting that PNA-specific glycans are intermittently masked by sialylation during thymic selection processes. Here, we studied expression of PNA receptors on developing thymocytes in the murine system using thymocytes from both normal mice and transgenic mice that are genetically arrested at the early phases of T cell development. Our results confirm and extend recent findings in the human system by showing that murine T cells sequentially progress from PNAlo-->PNAhi-->PNAlo stages during their differentiation within the thymus. In addition, our data demonstrate that a similar set of polypeptides is variably masked by sialylation throughout T cell development.

Surface molecules that drive T cell development in vitro in the absence of thymic epithelium and in the absence of lineage-specific signals

Differentiation of immature double positive (DP) CD4+ CD8+ thymocytes into single positive (SP) CD4+ and CD8+ T cells is referred to as positive selection and requires physical contact with thymic cortical epithelium. We now have identified "coinducer" molecules on DP thymocytes that, together with TCR, signal DP thymocytes to differentiate into SP T cells in vitro in the absence of thymic epithelium. A remarkable number of different molecules on DP thymocytes possessed "coinducing" activity, including CD2, CD5, CD24, CD28, CD49d, CD81, and TSA-1. Interestingly, in vitro differentiation occurred in the absence of lineage-specific signals, yet resulted in the selective generation of CD4+CD8- T cells. Thus, the present study has identified surface molecules that can signal DP thymocytes to differentiate into SP T cells in the absence of thymic epithelium and has characterized a default pathway for CD4+ T cell differentiation.

Lineage commitment in the thymus: only the most differentiated (TCRhibcl-2hi) subset of CD4+CD8+ thymocytes has selectively terminated CD4 or CD8 synthesis

Lineage commitment is a developmental process by which individual CD4+CD8+ (double positive, DP) thymocytes make a decision to differentiate into either CD4+ or CD8+ T cells. However, the molecular event(s) that defines lineage commitment is controversial. We have previously proposed that lineage commitment in DP thymocytes can be molecularly defined as the selective termination of CD4 or CD8 coreceptor synthesis. The present study supports such a molecular definition by showing that termination of either CD4 or CD8 synthesis is a highly regulated event that is only evident within the most differentiated DP subset (CD5hiCD69hiTCRhibcl-2hi). In fact, essentially all cells within this DP subset actively synthesize only one coreceptor molecule. In addition, the present results identify three distinct sub-populations of DP thymocytes that define the developmental progression of the lineage commitment process and demonstrate that lineage commitment is coincident with upregulation of TCR and bcl-2. Thus, this study supports a molecular definition of lineage commitment and uniquely identifies TCRhibcl-2hi DP thymocytes as cells that are already committed to either the CD4 or CD8 T cell lineage.

Identification of CD8 as a peanut agglutinin (PNA) receptor molecule on immature thymocytes

Differentiation of most T lymphocytes occurs within the thymus and is characterized by variable expression of CD4/CD8 coreceptor molecules, increased surface density of T cell antigen receptor (TCR) alpha beta proteins, and decreased expression of glycan chains recognized by the galactose-specific lectin peanut agglutinin (PNA). Although appreciated for several decades that PNA agglutination is useful for the physical separation of immature and mature thymocyte sub-populations, the identity of specific PNA-binding glycoproteins expressed on immature thymocytes remains to be determined. In the current report, we studied the expression of PNA-specific glycans on immature and mature T cells and used lectin affinity chromatography and immunoprecipitation techniques to characterize PNA-binding glycoproteins on thymocytes. Our data demonstrate that PNA-specific glycans are localized on a relatively small subset of thymocyte surface proteins, several of which were specifically identified, including CD43, CD45, and suprisingly, CD8 molecules. CD8 alpha and CD8 alpha' proteins bound to PNA in the absence of CD8 beta expression showing that O-glycans on CD8 beta glycoproteins are not necessary for PNA binding and that glycosylation of CD8 alpha and CD8 alpha' proteins proceeds effectively in the absence of CD8 beta. Finally, we demonstrate that PNA binding of CD8 is developmentally regulated by sialic acid addition as CD8 proteins from mature T cells bound to PNA only after sialidase treatment. These studies identify CD8 as a PNA receptor molecule on immature thymocytes and show that PNA binding of CD8 on immature and mature T cells is developmentally regulated by sialic acid modification.

Primary non-Hodgkin's lymphoma of the CNS treated with BVAM or CHOD/BVAM chemotherapy before radiotherapy

PURPOSE

To assess whether chemotherapy that includes drugs that cross the blood-brain barrier improves survival in primary CNS non-Hodgkin's lymphoma (PCNSL) when combined with radiotherapy.

PATIENTS AND METHODS

Thirty-four patients, with no evidence of human immunodeficiency virus type 1 (HIV-1) infection, were treated with the related chemotherapy regimens of carmustine (BCNU), vincristine, cytarabine, and methotrexate (BVAM; 12 patients), cyclophosphamide, doxorubicin, vincristine, and dexamethasone (CHOD)/BVAM (17 patients) and intensified CHOD/BVAM (five patients) between 1986 and 1994. The median age was 60 years (range, 16 to 73) and 47% had a performance status of 3 or 4 (Eastern Cooperative Oncology Group [ECOG]/World Health Organization [WHO]). Ten patients were treated with BVAM chemotherapy between 1986 and 1989, and subsequently 17 patients were treated with CHOD/BVAM (cytarabine 3 g/m2). Twenty of these 27 patients received whole-brain radiotherapy (craniospinal in four).

RESULTS

The complete response (CR) rate at the completion of chemotherapy was 63% for BVAM and 67% for CHOD/BVAM; more neutropenia occurred with CHOD/BVAM. The 5-year actuarial probability of survival of all 34 patients was 33% (95% confidence interval [CI], 14% to 52%), with so far only one recurrence after 2 years. Using multivariate analysis, age (P = .0005) and number of tumors at diagnosis (P = .0358) were prognostic factors. All five patients aged > or = 70 years died during or shortly after chemotherapy. Performance status was not an independent variable.

CONCLUSION

The BVAM or CHOD/BVAM regimens can be delivered despite neutropenia without significant treatment delay or dose reduction in patients less than 70 years of age. Further intensification of this type of chemotherapy is probably not possible with patients of this age, many of whom have a poor performance status.

Identification of Kv1.1 expression by murine CD4-CD8- thymocytes. A role for voltage-dependent K+ channels in murine thymocyte development

The patch-clamp recording technique and RNA-polymerase chain reaction were used to identify the voltage-dependent K+ channels expressed by murine fetal and adult CD4-CD8- thymocytes. Two distinct currents, encoded by the genes Kv1.1 and Kv1.3 were identified based upon their biophysical and pharmacologic characteristics and confirmed with RNA-polymerase chain reaction. Peptide blockers of Kv1.1 and Kv1.3 gene products were also applied to a murine fetal thymic organ culture system to investigate the developmental role of these K+ channels. Dendrotoxin (DTX) and charybdotoxin (CTX), antagonists of Kv1.1 and Kv1.3 channels, respectively, decreased thymocyte yields in organ culture without affecting thymocyte viability. DTX-treated thymi contained 56 +/- 8% (n = 8 experiments), and CTX-treated thymi contained 74 +/- 4% (n = 7 experiments) as many thymocytes as untreated lobes. DTX and CTX also altered the developmental progression of thymocytes in fetal organ culture. These data provide the first evidence of Kv1.1 expression in a lymphoid cell and indicate that thymocyte voltage-dependent K+ channels are critical to thymocyte preclonal expansion and/or maturation.

Asymmetric signaling requirements for thymocyte commitment to the CD4+ versus CD8+ T cell lineages: a new perspective on thymic commitment and selection

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ CD8- and CD4-CD8+ T cells requires that synthesis of one or the other coreceptor molecule be terminated, a process referred to as lineage commitment. The present study has utilized a novel coreceptor reexpression assay to identify lineage commitment in immature thymocytes and has found that the MHC recognition requirements for CD4 commitment and CD8 commitment fundamentally differ from one another. Remarkably, we found that thymocyte commitment to the CD8+ lineage requires MHC class I-dependent instructional signals, whereas thymocyte commitment to the CD4+ lineage is MHC independent and may occur by default. In addition, an unanticipated relationship between lineage commitment and surface phenotype has been identified. These results are incompatible with current concepts and require a new perspective on lineage commitment and positive selection, which we refer to as asymmetric commitment.

Early molecular events induced by T cell receptor (TCR) signaling in immature CD4+ CD8+ thymocytes: increased synthesis of TCR-alpha protein is an early response to TCR signaling that compensates for TCR-alpha instability, improves TCR assembly, and parallels other indicators of positive selection

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ or CD8+ T cells occurs within the thymus and is dependent upon expression of antigen receptor complexes (T cell receptor [TCR]) containing clonotypic alpha/beta proteins. We have recently found that CD4+ CD8+ thymocytes express low levels of surface TCR because of limitations placed on TCR assembly by the instability of nascent TCR-alpha proteins within the endoplasmic reticulum (ER) of immature thymocytes. Because TCR-alpha/beta expression increases during development, a molecular mechanism must exist for increasing the number of assembled TCR complexes present in immature CD4+ CD8+ thymocytes that have been signaled to differentiate into mature T cells, although no such mechanism has yet been described. In the current report we have examined the molecular consequences of intracellular signals generated by engagement of surface TCR complexes on immature CD4+ CD8+ thymocytes. Isolated TCR engagement generated signals that increased TCR-alpha RNA levels and increased synthesis of TCR-alpha proteins, which, in turn, significantly increased assembly of complete TCR-alpha/beta complexes in CD4+ CD8+ thymocytes. Increased TCR-alpha protein levels in TCR-signaled CD4+ CD8+ thymocytes was the result of increased synthesis and not increased stability of TCR-alpha proteins, indicating that TCR engagement compensates for, but does not correct, the inherent instability of TCR-alpha proteins in the ER of immature thymocytes. Consistent with the delivery by TCR engagement of a positive selection signal, TCR engagement also increased CD5 expression, decreased RAG-1 expression, and decreased CD4/CD8 coreceptor expression in immature CD4+ CD8+ thymocytes. These data identify amplified TCR-alpha expression as an initial response of immature CD4+ CD8+ thymocytes to TCR-mediated positive selection signals and provide a molecular basis for increased surface TCR density on developing thymocytes undergoing selection events within the thymus.

Stoichiometry of the T cell antigen receptor (TCR) complex: each TCR/CD3 complex contains one TCR alpha, one TCR beta, and two CD3 epsilon chains

The stoichiometry of the subunits that comprise the T cell antigen receptor (TCR) complex is not completely known. In particular, it is uncertain whether TCR alpha and TCR beta proteins are present in the TCR complex as one or multiple heterodimeric pairs. In this study we have used mice transgenic for two different TCR alpha and two different TCR beta proteins to determine the number of TCR alpha and TCR beta chains in a single TCR complex. Individual thymocytes and splenic T cells from double TCR transgenic mice simultaneously expressed all four transgenic TCR proteins on their surfaces. Because the individual TCR alpha and individual TCR beta proteins were biochemically distinguishable, we were able to examine association among the transgenic TCR products. We found that each TCR alpha chain paired with each TCR beta chain, but that each TCR complex contained only one TCR alpha and one TCR beta protein. Furthermore, quantitative immunofluorescence revealed that T cells expressed twice as many CD3 epsilon as TCR beta proteins. These findings demonstrate that there are precisely one TCR alpha, one TCR beta, and two CD3 epsilon chains in each TCR/CD3 complex expressed on the surfaces of both thymocytes and mature T cells.

Sudden contralateral deafness following cerebellopontine angle tumor surgery

Sudden hearing loss in the contralateral ear is a rare but catastrophic complication of acoustic tumor surgery. The seventh case in the literature is reported. Hearing can return, but this is less likely when the tumor was large or a suboccipital approach was used. A short course of corticosteroids, carbogen, and histamine cannot be shown to be effective but is probably worthy of consideration. Cochlear implantation may have a role if irreversible hearing loss persists 18 months after surgery.

Transition from TCR-beta dimer to TCR-alpha beta-expressing cells by introduction of an alpha-chain in an immature thymocyte cell line

The molecular basis for the surface expression of TCR-beta chain in the absence of association with TCR-alpha, -gamma, or -delta chain by an immature thymocyte cell line was investigated. The TCR-beta chain expressed by this cell line was not encoded by any unique DNA sequence, nor was it inserted into the membrane via a glycosyl-phosphatidylinositol linkage. Transfection of two other beta-chains derived from mature T cell clones resulted in the surface expression of dimers of the transfected beta-chains in both cases. Immunoprecipitation of the beta-dimer-CD3 complex demonstrated that the association of the beta-dimer with the CD3 complex, especially the CD3 zeta chain, was so weak that they dissociated under the detergent conditions in which the TCR-CD3 complex of mature T cells is kept intact. Transfection of TCR-alpha chain resulted in the expression of a TCR-alpha beta-CD3 complex and the disappearance of beta-dimers. In accordance with the changes in TCR complex components, the association between TCR-alpha beta and CD3 complex became stable and the cells transduced signals more efficiently. The results demonstrate that the expression of TCR-beta as part of an incomplete TCR-CD3 complex is developmentally regulated and the expression of TCR-alpha chain results in normal configuration and function of TCR complex.

Transition from TCR-beta dimer to TCR-alpha beta-expressing cells by introduction of an alpha-chain in an immature thymocyte cell line

The molecular basis for the surface expression of TCR-beta chain in the absence of association with TCR-alpha, -gamma, or -delta chain by an immature thymocyte cell line was investigated. The TCR-beta chain expressed by this cell line was not encoded by any unique DNA sequence, nor was it inserted into the membrane via a glycosyl-phosphatidylinositol linkage. Transfection of two other beta-chains derived from mature T cell clones resulted in the surface expression of dimers of the transfected beta-chains in both cases. Immunoprecipitation of the beta-dimer-CD3 complex demonstrated that the association of the beta-dimer with the CD3 complex, especially the CD3 zeta chain, was so weak that they dissociated under the detergent conditions in which the TCR-CD3 complex of mature T cells is kept intact. Transfection of TCR-alpha chain resulted in the expression of a TCR-alpha beta-CD3 complex and the disappearance of beta-dimers. In accordance with the changes in TCR complex components, the association between TCR-alpha beta and CD3 complex became stable and the cells transduced signals more efficiently. The results demonstrate that the expression of TCR-beta as part of an incomplete TCR-CD3 complex is developmentally regulated and the expression of TCR-alpha chain results in normal configuration and function of TCR complex.

Negative selection of CD4+CD8+ thymocytes by T cell receptor-induced apoptosis requires a costimulatory signal that can be provided by CD28

CD4+CD8+ thymocytes expressing self-reactive T cell antigen receptors (TCR) are deleted in the thymus as a consequence of TCR/self-antigen/major histocompatibility complex interactions. However, the signals that are necessary to initiate clonal deletion have not yet been clarified. Here we demonstrate that TCR engagement does not efficiently induce apoptosis of CD4+CD8+ thymocytes, although it generates signals that increase expression of CD5, a thymocyte differentiation marker. In fact, TCR signals fail to induce thymocyte apoptosis even when augmented by simultaneous engagement with CD4 or lymphocyte function 1-associated molecules. In marked contrast, signals generated by engagement of both TCR and the costimulatory molecule CD28 potently induce apoptosis of CD4+CD8+ thymocytes. Thus, the present results define a requirement for both TCR and costimulatory signals for thymocyte apoptosis and identify CD28 as one molecule that is capable of providing the necessary costimulus. These results provide a molecular basis for differences among cell types in their ability to mediate negative selection of developing thymocytes.

CD4+/CD8- thymocytes dominate the fetal thymus treated with a combination of anti-T cell receptor-beta and anti-CD4 antibodies

Two major phenotypic changes characterize the development of a mature thymocyte from its CD4+/CD8+ (double positive or DP) precursor: the loss of expression of either CD4 or CD8 and the increase in the level of surface TCR. The specific surface interactions responsible for these changes are unknown, but studies using the fetal thymus as an experimental system have provided clues by identifying conditions that alter these maturational events. Development to the CD4+/CD8-/TCR-alpha beta high (single positive) phenotype is inhibited when thymocytes in fetal organ culture are exposed to antibodies directed against the CD4 molecule, the CD3 complex or the TCR-alpha/beta heterodimer. We show in this study, however, that treatment of fetal thymic lobes with a combination of anti-CD4 and anti-TCR-beta antibodies results in a marked increase in the proportion of mature CD4+/CD8-/TCR-alpha beta+ thymocytes and a decrease in the proportion of DP thymocytes. Although treatment of lobes with a combination of anti-CD4 and anti-CD3 epsilon antibodies also depletes cultures of DP thymocytes, the CD4+/CD8-/TCR+ population does not develop. Our results are consistent with the hypothesis that coengagement of CD4 and TCR biases development to the CD4 single positive phenotype and with observations that TCR engagement and CD3 engagement have different developmental consequences.

CD4+/CD8- thymocytes dominate the fetal thymus treated with a combination of anti-T cell receptor-beta and anti-CD4 antibodies

Two major phenotypic changes characterize the development of a mature thymocyte from its CD4+/CD8+ (double positive or DP) precursor: the loss of expression of either CD4 or CD8 and the increase in the level of surface TCR. The specific surface interactions responsible for these changes are unknown, but studies using the fetal thymus as an experimental system have provided clues by identifying conditions that alter these maturational events. Development to the CD4+/CD8-/TCR-alpha beta high (single positive) phenotype is inhibited when thymocytes in fetal organ culture are exposed to antibodies directed against the CD4 molecule, the CD3 complex or the TCR-alpha/beta heterodimer. We show in this study, however, that treatment of fetal thymic lobes with a combination of anti-CD4 and anti-TCR-beta antibodies results in a marked increase in the proportion of mature CD4+/CD8-/TCR-alpha beta+ thymocytes and a decrease in the proportion of DP thymocytes. Although treatment of lobes with a combination of anti-CD4 and anti-CD3 epsilon antibodies also depletes cultures of DP thymocytes, the CD4+/CD8-/TCR+ population does not develop. Our results are consistent with the hypothesis that coengagement of CD4 and TCR biases development to the CD4 single positive phenotype and with observations that TCR engagement and CD3 engagement have different developmental consequences.

Management and outcome of severe head injuries in the Trent region 1985-90

In a five year period, 39 children (29 boys, 10 girls) aged 2 months to 13 years (mean 7.8 years) were studied who had suffered a major head injury (29 road traffic accidents, six falls, and four non-accidental injury). The injury had been assessed clinically and by cranial computed tomography or cranial ultrasound (in a single baby of 2 months). Initial Glasgow coma scores for all subjects ranged from 3-11 (mean 5.5), intact survivors 5-11 (7.4), minor handicap 4-11 (6.1), major handicap 3-6 (4.3), fatalities 3-6 (4.1). All were treated with sedation, paralysis, hyperventilation (arterial carbon dioxide tension 3.0-3.5 kPa), intracranial pressure monitoring and moderate body surface hypothermia to 32 degrees C. Nine children died and 30 survived (nine intact, 13 minor disability, and eight major disability). The worst cerebral perfusion pressure was over 40 mm Hg in all but one survivor, and less than 40 mm Hg in seven of nine fatalities. Severe hypocapnia both in the first 24 hours and overall was correlated with poor outcomes (dead or major disability), as were bilateral contusions or diffuse axonal injury.

Cerebral contusional tears as a marker of child abuse--detection by cranial sonography

A series of 6 infants subjected to child abuse is presented in whom contusional tears of subcortical white matter were detected during life by intracranial sonography. The sonographic appearances of this highly pathognomonic marker of shaking injury are described for the first time and their significance discussed. On the basis of our experience we suggest that high resolution cranial sonography is an extremely valuable part of the diagnostic work up in cases of suspected non-accidental injury.

Surface expression of a T cell receptor beta (TCR-beta) chain in the absence of TCR-alpha, -delta, and -gamma proteins

The antigen receptor expressed by mature T cells has been described as a disulfide-linked alpha/beta or gamma/delta heterodimer noncovalently associated with CD3, a complex of transmembrane proteins that communicates signals from the T cell receptor (TCR) to the cell interior. Studies suggest that all component chains must assemble intracellularly before surface expression can be achieved. We described, however, a CD4+/CD8+ transformed murine thymocyte, KKF, that expresses surface TCR-beta chains in the absence of gamma, delta, and alpha proteins; these beta chains are only weakly associated with CD3-epsilon and CD3-zeta. Furthermore, KKF responds differently to stimulation through TCR-beta and CD3-epsilon, a functional dissociation that has been ascribed to a CD4+/CD8+ subpopulation of normal thymocytes. KKF's unique TCR structure may offer an explanation for the functional anomalies observed.