Differentiation from precursors in athymic nude mouse bone marrow of unusual spontaneously cytolytic cells showing anti-self-H-2 specificity and bearing T cell markers

Cartilage regeneration by selected chondrogenic clonal mesenchymal stem cells in the collagenase-induced monkey osteoarthritis model

Osteoarthritis (OA) is the most common form of arthritis, in which cartilage is irreversibly degraded, causing severe pain and disability. Current therapeutic strategies cannot repair damaged cartilage. We evaluated the repair potential of selected chondrogenic clonal MSCs (sC-MSCs) by delivering them into the injured cartilage site in a collagenase-induced OA model in Cynomolgus monkeys. In vitro characterization showed that the isolated monkey sC-MSCs and polyclonal MSCs (P-MSCs) expressed mesenchymal stem cell markers and could differentiate into chondrocytes. The articular cartilage lesions in animals were treated with normal saline (NS), autologous P-MSCs and sC-MSCs, respectively, by direct delivery. The clinical parameters, radiographic images, histological and immunohistochemical examinations at weeks 8, 16 and 24 post-treatment demonstrated that the abrasions of articular cartilage were significantly improved and repaired by MSC-based treatment, particularly in the sC-MSC-treated group, which displayed consistently higher histological scores than those of other groups. In summary, treatment with sC-MSCs can effectively improve the healing of cartilage lesions in the Cynomolgus monkey collagenase-induced OA model. Due to the genetic proximity of monkey and human, the therapeutic strategy presented in this study will have broad applications in clinical practice.

Diversity of functional T-cell receptor delta-chain transcripts from bone marrow cells of athymic nude mice

CD3+ cells are detectable in bone marrow of athymic mice homozygous for the nude mutation. As previously shown, cells expressing the gamma delta T-cell receptor (TcR) represent 30-40% of this T-cell population. Using V delta-specific, V alpha 4-specific, and C delta-specific primers, TcR delta-chain transcripts were reverse transcribed and polymerase chain reaction (PCR)-amplified from total RNA prepared from bone marrow cells (BMC) of 6-month-old NMRI nu/nu mice. Amplified TcR delta-chain cDNA was cloned, and 49 randomly selected clones derived from seven amplification reactions were sequenced. Sequence analyses showed: (1) more than 80% of the sequenced clones represented in-frame transcripts of the TcR delta-chain; (2) in-frame transcripts containing V delta 1-, V delta 2-, V delta 3-, V delta 4-, V delta 5-, V delta 6- and V alpha 4-gene segments were detectable in nude BMC; (3) V delta 2-, V delta 4- and V delta 5-containing transcripts were more abundant and more diverse than V delta 1- and V delta 3-containing transcripts; (4) extensive N-region diversity was present in the V delta-D delta 2 (N1), D delta 2-D delta 1 (N2) and D delta 1-J delta 1 (N3) junctional regions; (5) P nucleotide additions were present in many transcripts; and (6) unusual truncated, in-frame transcripts with deleted D- and J-region genes were detected. A large potential TcR delta-chain repertoire is thus present in nude BMC.

Bone marrow cells of athymic nude mice express functional T cell receptor alpha chain transcripts rearranged to V delta 2, 3, 4, 5, 6 genes

From bone marrow cells (BMC) of athymic nude mice, T cell receptor (TcR) alpha chain transcripts were selectively amplified by polymerase chain reaction (PCR) using V delta 2-, V delta 3-, V delta 4-, V delta 5-, V delta 6- and C alpha-specific primers. Amplified DNA fragments were cloned, and 32 randomly selected clones from 5 PCR were sequenced. Twenty-three distinct rearrangement events were detected, of which 87% (20/23) were in-frame. All five tested V delta genes (V delta 2, 3, 4, 5, 6) rearranged in-frame to J alpha-C alpha. N-region diversity in V delta-J alpha junctions present in most clones was limited to two to five nucleotides. P-nucleotide additions in this region were also detected. The V delta 5 gene located 3' of C delta in reversed transcriptional orientation was rearranged to J alpha by inversion. The J alpha usage pattern of the sequenced clones was strongly biased towards rearrangement of the most 5' genes (located nearest to C delta) of the J alpha cluster: the most 5' J alpha (J alpha TA1) was used by 30% of all clones, and 78% of all J alpha rearranged to V delta were located in the 5' 12 kb of the 60-kb J alpha cluster. As distinct V delta/C delta and V alpha/C alpha TcR usage patterns are prevalent in peripheral T cell populations, our data suggest that these TcR usage patterns results from repertoire selections operating in alpha beta and gamma delta T cell lineages, but not from preferential V delta-C delta and V alpha-C alpha rearrangement patterns.

Thymic epithelial cell-derived supernatants sustain the maturation of human prothymocytes: involvement of interleukin 1 and CD23

During their development, human CD7+ lymphoid stem cells migrate into the thymus where, following intimate contact with thymic tissue, they proliferate and differentiate into functionally mature T lymphocytes. In this study, we investigated the effect of thymic epithelial cell-derived supernatants (TEC-SN) on early CD7+CD2-CD3- thymocytes. Our results indicate that TEC-SN are able to promote CD2 and CD3/TcR alpha/beta expression by CD7+ precursors. This activity correlated with soluble CD23 and interleukin 1 levels in TEC-SN. Furthermore, monoclonal antibodies to these cytokines decreased in vitro maturation of prothymocytes. Thus, in addition to cell-cell interactions, human TEC produce cytokines able to support early steps of thymocyte differentiation.

Thymectomy at weaning. An accelerated aging model for the mouse immune system

Mouse thymectomy at weaning induces a long lasting immunodepression which can be measured by in vivo and in vitro experiments. Lymphocyte proliferation and IL2 production in response to a T cell mitogen are greatly diminished during the whole life of the animals, on the contrary B cell proliferation in the presence of lipopolysaccharide is not modified. The lack of effect of surgery on the in vitro T cytotoxic activity compared to the total abolition of in vivo graft versus host reaction shows that these two phenomena are under the control of different immunocompetent cell subsets. Thymectomy induces a stabilization of natural killer cell activity, while during normal aging, this parameter decreases regularly. Surprisingly, the thy 1+ cell level is normal 8-10 months after thymectomy compared to sham operated animals showing that phenotypically normal cells can be dysfunctional. Macrophage activity is not modified either by aging or by thymectomy. So, thymectomized mice can be used after less than 1 year to study immunopharmacology of aging.

Development of T cells in vitro from precursors in mouse bone marrow

Bone marrow cells from 6- to 8-week-old athymic nude mice were depleted of nylon-wool adherent cells and cultured in vitro at low cell numbers (300 cells/well) in medium supplemented with a supernatant from a thymoma cell line. About 1% of cultured cells grew. Pooled cultures contained cells expressing CD3 (52%), CD4 (37%), CD8 (11%), Thy 1.2 (72%), MAC-1 (43%) and J11d (86%) but no cells expressing sIg. They also contained cells expressing mRNA for the alpha, beta, gamma, and delta chains of the T cell receptor as assessed with C region probes using a sensitive dot blot assay. These cells appear to develop from progenitors which are CD3-. When pooled Day 10 cultures were depleted of nylon-wool adherent cells, the remaining cells were nearly all J11d+, Thy 1.2+, MAC-1-, CD3+, and either CD4+CD8+; CD4+CD8-; CD4-CD8+, or CD4-CD8-; i.e., their surface marker patterns were reminiscent of those of thymocytes. We conclude that our culture system is enabling bone marrow precursors to commence differentiation down the T cell lineage in the absence of a thymic environment.

Development of autoreactive L3T4+ T cells from double-negative (L3T4-/Ly-2-) Thy-1+ spleen cells of normal mice

Thy-1+/L3T4-/Ly-2- spleen cells were purified from normal C57BL/6 (B6) and C,B-17 mice. Cells within this subset expressed the T cell receptor (TcR) for antigen: the majority of cells in this subset were CD3+; a fraction of the cells was stained with the monoclonal antibody (mAb) F23.1; and the TcR molecule was immunoprecipitable with mAb F23.1 from cells within this subset. In limiting dilution analyses, about 1/30 cells within this subset were growth inducible in vitro by stimulation with phorbol myristate acetate (PMA) plus ionomycin; conditioned media containing interleukin (IL) 1, IL2, IL3 or IL4 activity neither triggered nor promoted in vitro growth of these cells. The in vitro generated T cells displayed the Thy-1+/L3T4+/Ly-2- surface phenotype and were self-reactive, i.e., proliferated preferentially in response to syngeneic stimulator cells, and secreted IL2 and IL3 only in response to syngeneic but not allogeneic stimulator cells. The proliferative response of these cells to syngeneic stimulator cells was blocked by anti-self Ia mAb. This autoreactive helper T cell subset was not inducible in purified Thy-1+ spleen cell subsets from athymic nude mice or scid mice. Autoreactive helper T cells did not express detectable levels of the IL2 receptor (IL2R), and their proliferative response was not blocked by anti-IL2R mAb. From PMA plus ionomycin-stimulated double-negative Thy-1+ spleen cells, 14 T cell clones were established in long-term culture which displayed the CD3+CD4+CD8- surface phenotype and were self-reactive.

The autoimmune mouse MRL/Mp-lpr/lpr contains cells with spontaneous cytotoxic activity against target cells bearing self-determinants

Spontaneously cytotoxic cells possessing specificity and having a complex pattern of reactivity directed at least partly against self-determinants develop by the age of 10 weeks in the autoimmune mouse strain MRL/Mp-lpr/lpr (MRL-lpr) and by the age of 6 months in C57BL/6-lprl/lpr. Similar effector cells do not develop in either MRL/Mp-+/+(MRL-+/+) or normal C57BL/6 mice up to 6 months old. Freshly prepared suspensions of both lymph node and bone marrow cells from individual MRL-lpr mice could kill Con A- or LPS-induced blast cells and fresh thymocytes from MRL-+/+ and other mouse strains with strong preference for strains carrying (self)-H-2k determinants in a 4-hr51Cr release assay. These results imply that self-reactive cells are generated as part of the lpr gene defect.

Progression of rearrangements at T cell receptor beta and gamma gene loci during athymic differentiation of bone marrow cells in vitro

A previously described in vitro system that supports T cell differentiation from bone marrow (BM) precursors was analyzed for T cell receptor gene rearrangement and expression. Cultured populations from days (d.) 0, 3, 6, and 12-13 were fused with BW5147. Only hybridomas from d.6 and d.12-13 bore BM-derived rearrangements. The rearrangement complexity was higher on d.12 than d.6 in that patterns consistent with D beta 1-J beta 1 and D beta 2-germ-line configurations decreased while V-D-J beta 2 and V gamma 2-J gamma 1 joining became dominant. Northern blots of d.13 BM cultures revealed gamma, alpha, and beta (1.0 and 1.3 kb) transcripts. Parallels between patterns in BM cultures and thymus ontogeny were recognized.

Colony formation by subpopulations of human T lymphocytes. VI. Further studies on colony phenotype, function, and cloning efficiency

Phytohemagglutinin (PHA)-induced colony formation in semisolid agar medium by human peripheral blood T lymphocytes showed an increasing cloning efficiency with decreasing numbers of cultured cells. Ninety percent of CD4+ cells (inducer/helper phenotype) and 20% of CD8+ cells (cytotoxic/suppressor phenotype) formed colonies when cultured at 10-200 cells/ml culture in the presence of sheep red blood cells (SRBC) and a source of interleukin-2 (IL-2). Probably all T-colony-forming cells, but none of the subsequent colony cells, expressed the Leu-8 antigen. The cloning efficiencies of FACS-sorted cells expressing the natural killer antigenic phenotypes Leu-7+ and CD16+ were found to be less than 1%. The costimulatory effect of red blood cells for colony formation was specific for SRBC and not observed in the presence of red cells obtained from seven other species including man. All T-lymphocyte colonies obtained from unseparated peripheral blood mononuclear cells expressed the CD25 antigen (IL-2 receptor) and colonies were always composed of either CD4+ or CD8+ cells. None of the colony cells expressed the Leu-8 or the CD16 antigens. By their specific morphology in agar culture the majority of colonies composed of CD4+ cells were easily recognized, but but approximately one-third of the CD4+ colonies could not be distinguished from colonies composed of CD8+ cells. On expansion of individual colonies in liquid subculture in the presence of interleukin-2, approximately 15% of the colonies developed natural killer (NK)-like cytotoxic activity, being capable of direct killing of K562 tumor cells. It is concluded that the present method for growing human T colonies exhibits the same cloning efficiency as the most efficient liquid culture systems. Individual T colonies are composed exclusively of T inducer/helper or T cytotoxic/suppressor cells, they are never of mixed phenotype, and they do not contain cells of natural killer phenotype. Regulatory mechanisms influencing colony formation are operating between and within the various subsets of T lymphocytes.

A programmed functional and phenotypic development of bone marrow-derived cytotoxic cell precursors in vitro

An analysis of function and marker expression during cytotoxic cell differentiation in vitro from T cell-depleted bone marrow precursors is described. Cytotoxic activity is not detectable during the first five days of culture, but rises abruptly soon after. Antibody plus complement depletion studies show that cytotoxic cells derive from Thy-1-negative precursors and undergo a continual increase in Thy-1 and Lyt-2 marker expression as the culture progresses. A reciprocal decrease in asialo-GM1 antigen expression on effector cells is seen. The J11d-negative precursor cells acquire J11d (an antigen known to mark cortical thymocytes) at an intermediate stage of culture, but revert to the J11d-negative phenotype prior to functional acquisition. At least some effectors are T cell receptor positive as illustrated by an anti-T cell receptor antibody-mediated killing assay. These patterns precisely correlate with those detected among developing T cells in vivo. Results may indicate that a programmed course of differentiation inherent to bone marrow cells may be triggered in the absence of a thymic environment.

Specificity repertoire of splenic Lyt-2+/F23+ cytotoxic lymphocyte precursors from B6 mice

As revealed by flow cytometric analysis, about 30% of nylon wool nonadherent Lyt-2+ B6 spleen cells were F23+, i.e., were stained with the monoclonal antibody F23.1 directed against an allotypic T-cell receptor determinant. The specificity repertoire of splenic Lyt-2+/F23+ cytotoxic lymphocyte precursors (CLP) from B6 mice was investigated in a limiting dilution (LD) system designed to support clonal expansion in vitro of a representative fraction of this T-cell subset: in highly purified Lyt-2+ responder cells cocultured with mitomycin-treated F23 hybridoma cells in the presence of (recombinant) interleukin 2 under LD conditions, one out of three Lyt-2+/F23+ CLP gave rise to a functional cytotoxic T lymphocyte (CTL) clone. The split-well analysis of individual CTL populations demonstrated a clear-cut segregation of the lytic reactivities toward different allogeneic Con A blast targets. A large fraction of B6-derived CTL clones (3-10%) specifically lysed fully H-2 allogeneic (H-2k, H-2d), or H-2K mutant (bm1) targets. Self-reactive and allorestricted lytic patterns were not found.

Phenotypic characterization of murine lymphokine-activated killer cells

Short-term culture of murine lymphocytes in interleukin 2 (IL-2), in the absence of any priming antigen, has been shown to result in the differentiation of an activated killer cell population capable of potent cytotoxic activity against tumor cells. The progenitor and lineage of these lymphokine activated killer cells (LAK) remains controversial. The present study was initiated to combine both complement-mediated depletion and flow cytometry to examine the cell surface membrane markers on murine LAK precursors and effectors. Selective depletion of antigen-positive cells from the precursor or effector population followed by functional assays demonstrates that the LAK effector is derived from a non-thymus-processed cell (Thy-1 negative). Paradoxically, the effector acquires Thy-1 expression in parallel to the IL-2 induced acquisition of killer cell effector function. These studies clearly show that both precursor and effector cells express the "NK-associated" Qa 5 and asialo GM-1 surface antigens. Mature effectors, but not the precursors, exhibit both Lyt-2 and the "NK-associated" NK-1.1 cell surface marker. Our flow cytometric analyses of murine spleen cells activated in rIL-2 have identified a distinct large, granular cell population which contains the LAK effector. This population, which can be readily discerned using light scattering properties with a flow cytometer, demonstrates both quantitative and qualitative changes in cell surface antigen expression.

The human lymphokine-activated killer cell system. V. Purified recombinant interleukin 2 activates cytotoxic lymphocytes which lyse both natural killer-resistant autologous and allogeneic tumors and trinitrophenyl-modified autologous peripheral blood lymphocytes

Culture of human peripheral blood lymphocytes (PBL) in purified natural or recombinant interleukin 2 in the absence of exogenous antigen or mitogen causes the differentiation of nonlytic precursor cells into lymphokine-activated killers (LAK). A titration of purified Jurkat IL-2 (BRMP, FCRC, NIH) IL-2 showed that the relatively low concentration of 5 U/ml was optimal for LAK activation. When the responding PBL were pretreated with either mitomycin C or gamma irradiation, LAK activation did not occur, indicating that proliferation, in addition to differentiation, is required. The spectrum of target cells susceptible to LAK lysis in a 4-hr chromium-51-release assay includes fresh NK-resistant tumor cells and trinitrophenyl (TNP)-modified autologous PBL. Unmodified PBL are not lysed. Cold target inhibition studies indicated that LAK lysis of autologous TNP-PBL is totally inhibited by fresh tumors cells, and that tumor lysis is inhibited by TNP-PBL. Additionally, allogeneic tumors totally inhibit lysis of autologous tumor cells in other cold target studies. These results demonstrate that the lytic activity expressed by LAK is not HLA restricted, is not limited to tumor cells, and is "polyspecific" as indicated by the cross-reactive recognition of multiple target cell types in these cold target inhibition studies.

Allorestricted cytotoxic T cells. Large numbers of allo-H-2Kb-restricted antihapten and antiviral cytotoxic T cell populations clonally develop in vitro from murine splenic precursor T cells

Cytotoxic T lymphocyte (CTL) responses of splenic T cells from C57BL/6 B6) mice and mutant H-2Kbm1 (bm1) mice to haptenic (trinitrophenyl [TNP] ) and herpes simplex virus (HSV) determinants in the context of an allogenic (wild-type or mutant) H-2Kb molecule were analyzed in a modified limiting dilution system. In the B6-anti-bm1TNP mixed leukocyte reaction (MLR), estimated frequencies for precursors of CTL clones that lysed bm1TNP targets ranged from 1/120 to 1/400; in the bm1-anti-B6TNP MLR, estimated frequencies of precursors of CTL clones that lysed B6TNP targets ranged from 1/500 to 1/1,300. Estimated frequencies for precursors of CTL clones that lysed the respective unmodified and TNP-modified allogeneic targets were two- to three-fold lower. Lytic specificity patterns determined by split-well analysis showed that at least 20-30% of the generated CTL populations (selected for a high probability of clonality) in both MLR displayed allorestricted lysis of TNP-modified concanavalin A blast targets. In the B6-anti-bm1HSV MLR, estimated frequencies for precursors of CTL clones that lysed bm1HSV targets ranged from 1/70 to 1/300; in the bm1-anti-B6HSV MLR, estimated frequencies for precursors of CTL clones that lysed B6HSV targets ranged from 1/300 to 1/1,200. Again, estimated frequencies for precursors of CTL clones that lysed the respective noninfected and virus-infected allogeneic targets were two- to fourfold lower. Of the CTL populations selected for a high probability of clonality at least 30-60% displayed allorestricted lysis of virus-infected lipopolysaccharide blast targets in both MLR. It is concluded that a large fraction of clonally developing CTL populations stimulated with TNP-modified or HSV-infected allo-H-2Kb-bearing cells displayed an allorestricted pattern of recognition. It was further evident that the estimated frequencies of splenic precursors that generated allorestricted CTL clones was two- to threefold higher than the estimated frequencies of precursors that gave rise to the respective alloreactive CTL populations.

Alloreactive cytotoxic T cells. I. Alloreactive and allorestricted cytotoxic T cells

Nylon wool-nonadherent spleen cells from three inbred mouse strains of H-2k (CBA), H-2d (BALB/c) and H-2b (C57BL/6) haplotype were co-cultured with 2,4,6-trinitrophenyl (TNP)-modified or nonmodified allogeneic stimulator cells in a limiting dilution system. Using a recently described restimulation protocol, a surprisingly large number of splenic cytotoxic lymphocyte precursors (CLP) was clonally expanded in this primary in vitro response to allo-H-2 plus TNP determinants; measured CLP frequencies ranged from 1/30 to 1/300. The lytic specificity patterns of individual microcultures (selected for a high probability of clonality) were defined by split well analysis, and were furthermore followed up in time by sequentially reassaying microcultures at different time points of in vitro incubation. This analysis revealed the following: a large fraction of cytotoxic T lymphocyte clones lysed TNP-modified but not nonmodified allogeneic concanavalin A blast targets, i.e., were allorestricted; this was found in all 6 allogeneic strain combinations set up with b, k and d haplotype mice; allorestricted lytic patterns predominated in microcultures with low numbers of responder cells per well, and at late time points of in vitro culture; allorestricted lytic cultures were specific for the stimulating allogeneic H-2 plus TNP determinant(s); and allorestricted lytic patterns were also found in microcultures stimulated by nonmodified allogeneic cells. To our knowledge, these are the highest CLP frequencies yet reported in limiting dilution systems that used a specific (re)stimulation protocol and measured the lytic responses obtained in a specificity-controlled readout.

Rapid changes in specificity within single clones of cytolytic effector cells

We find rapid changes in the specificity of the cytolytic effector cells in a mixed lymphocyte culture. The lysis patterns produced by cytolytic effector cells generated near limiting dilution in murine mixed lymphocyte reactions of three types, F1 anti-parent (F1(A X B) anti-A), allogeneic (C anti-F1(A X B)), and F1 antimodified parent (F1(A X B) anti-A-TNP), were investigated. Cultures were characterized by their ability or inability to lyse a panel of target cells (e.g., A, B, F1). When individual cultures were tested at two different times, changes in lytic pattern were routinely seen, with some patterns reproducibly increasing in frequency and others reproducibly decreasing (e.g., patterns involving lysis of F1 decreased in an F1 anti-A response but increased in a C anti-F1 response). X-linked isoenzyme analysis showed that changes can occur within a single clone of effector cells. These results imply that the T cell specificity repertoire continues to evolve during an ongoing immune response, a conclusion incompatible with clonal selection theory.

Functional heterogeneity in allospecific cytotoxic T lymphocyte clones. I. CTL clones express strong anti-self suppressive activity

Five out of five allo-specific cytotoxic T lymphocyte (CTL) clones tested strongly suppressed the development of CTLs directed against the H-2 haplotype of the CTL clone and independent of the H-2 specificity recognized by the CTL clone. This was shown by including 100-1,000 cells from the five clones in one way mixed lymphocyte reaction (MLR) cultures in which the stimulator cells were of the same H-2 type as the CTL cells. When these cultures were assayed for cytotoxicity against the stimulator cell haplotype, the cytotoxic activity was decreased in a CTL cell dose-dependent manner by 50 to more than 90%. Suppression was usually not observed in MLR cultures where the CTL-H-2 type was identical with the responder cells or was different from both the responder or stimulator cells. Suppression was demonstrated not to be due to "cold" target inhibition at the time of cytotoxicity assay. Even if the added CTL were completely removed after 48-72 h of culture, significant suppression was obtained. Suppressive ability did not appear to be correlated with the level of allo-specific cytotoxic activity present in the CTL clones, but might involve direct killing of MLR precursor cells by cells in the added CTL clones. The suppression observed here, which is anti-self from the point of view of the added CTL clone, appears to be triggered by precursor cells in the MLR responder population recognizing MHC determinants on cells from the added CTL clone. This peculiar type of suppression, in which the regulator regulates on being recognized, has been christened the veto phenomenon and may play a role in maintenance of self tolerance.

A selective culture system for generating terminal deoxynucleotidyl transferase-positive (TdT+) lymphoid precursor cells in vitro. I. Description of the culture system

A primary xenogeneic culture system has been devised that selectively generates undifferentiated TdT+ lymphoblasts from rat bone marrow under conditions that do not support the growth or maintenance of rat colony-forming unit-spleen (CFU-S) or granulocyte/macrophage colony-forming cells (GM-CFC). The culture system requires a mouse bone marrow feeder layer, and a serum supplement that has markedly reduced levels of cortisol. The growth of TdT+ cells can be significantly enhanced by the addition of mesodermalizing factors (e.g., fibroblast growth factor, guinea pig bone marrow extract) to the culture medium, and the serum supplement can be decreased by the addition of selenium, transferrin, and T3. The cultured TdT+ cells are antigenically "null" cells that further resemble their normal counterparts in bone marrow with respect to morphology, size, cortisone sensitivity, and pattern of TdT fluorescence. The TdT+ cells are generated with equal facility from bone marrow of normal and congenitally athymic rats, can be maintained in logarithmic growth for at least 10 mos by serial passage in vitro, and do not cause leukemia when infused into irradiated recipients. Although the lineage relationships of these immature lymphoid cells have not yet been established, our working hypothesis, based on preliminary evidence, is that the cultured TdT+ cells are primitive members of the T cell series.