TCR repertoire to proteolipid protein (PLP) in multiple sclerosis (MS): homologies between PLP-specific T cells and MS-associated T cells in TCR junctional sequences

In the pathogenesis of multiple sclerosis (MS), autoimmune Tcells reactive with proteolipid protein (PLP) may play a crucialrole. We determined 23 TCR (ß-chain sequences of limitingdilution T cell lines (TCL) selected against a synthetic peptide,PLP 95–116, 105–124 or 139–155, from the peripheralblood of three Japanese MS patients with the DR2, w15 haplotype(Tl, SK and OK). Fourteen sequences were originated from Tl,seven from SK and two from OK. The PLP-reactive TCL utilizedvarious V_ß and J^ß; gene segments, but therewas significant bias in the V_ß and J_ß usage.Overutilization of the V_ß2 family and dominant usageof the J_ß2.5 subfamily was seen in PLP 105–124-reactiveand 95–116-reactive TCL respectively. More remarkably,a majority of the TCL were found to express ß-chainCDR3 motifs that appear to be unique to MS brain infiltrates.In contrast, these motifs were only rarely seen in control TCRsequences from peripheral blood or from a TCL selected againsttetanus toxoid. In several cases, the _ßCDR3 homologiesbetween the PLP-reactive T cells and MS brain T cells were extensive,owing to the shared motifs in combination with the surroundingamino acid identities. These results indicate that PLP-specificT cells may be involved in the immunopathology of MS.     

T cell subpopulation phenotypes in filarial infections: CD27 negativity defines a population greatly enriched for Th2 cells

Three-color flow cytometric analysis was used to define surfacemarkers which identify the T_h2-type CD4⁺ cells responsible forthe eosinophilia and elevated serum IgE typical of tissue invasivehelminth infections. A group of six mAba to well known cellsurface markers were screened for differential expression onCD4⁺ CD45RO⁺ lymphocytes from normal individuals (NL; n = 6)and filaria-infected patients (PT; n = 10). The majority ofmarkers were expressed equally by both groups, but the CD4⁺CD45RO⁺ cells in the PTs showed significantly higher levelsof expression of HLA-DR than those of NLs (P = 0.014). ThisCD4⁺ HLA-DR⁺ subpopulation was then studied further for itsexpression of an additional 10 activation and adhesion molecules.CD27 showed a trend for lower intensities of expression on PTCD4⁺ HLA-DR⁺ cells than on those of NLs. Analysis of the serumfrom both NLs and PTs revealed that PTs had significantly higherlevels of soluble CD27 and CD25 (IL-2R) In the serum than NLs(P < 0.01 and P = 0.022 respectively) indicating a generalstate of Immune activation and differentiation. Functional analysisof the CD4⁺ HLA-DR⁺ and the CD4⁺ CD27^– subpopulatlonsrevealed that the CD4⁺ HLA-DR⁺ cells produced significantlyhigher levels of IL-5 than the CD4⁺ HLA-DR^– cells (P <0.04), and the CD4⁺ CD27^– cells produced significantlyhigher levels of both IL-4 and IL-5 than the CD4⁺ CD27^–cells (P <0.05 and P <0.001 respectively). Thus, whilethe CD4⁺ CD27^– and CD27⁺ subpopulatlons contain T_h1 andT_h0 cells, only the CD4⁺ CD27^– population contains theT_h2 cells (producing both IL-4 and IL-5).     

Co-stimulation via CD28 induces activation of a refractory subset of MRL-Ipr/Ipr T lymphocytes

Peripheral lymphoid tissues of Ipr mice contain a large proportionof TCRß/CD3⁺CD4^–CD8^– T cells that lacksurface CD2 and express the B cell isoform of CD45, B220. Thissubset of T cells does not proliferate or produce IL-2 in responseto mitogenic signals or TCR–CD3 ligation. At the sametime, these abnormal T cells display several characteristicsof an activated phenotype. Collectively, these properties ofIpr CD4^–CD8^– T cells have functional parallels withanergic T cells. A critical co-stimulatory molecule implicatedin the prevention of or recovery from anergy is CD28, whichbinds the ligand BB1/B7 on certain accessory cells. Ipr CD4^–CD8^–T cells express normal levels of CD28 which is capable of transducinga strong proliferative signal to these cells in co-stimulationwith mitogens. However, proliferation of Ipr CD4^–CD8^–T cells in response to CD28 co-stimulation does not reach thelevels observed in normal T cells stimulated under similar conditions.Stimulation with anti-CD28 mAb in conjunction with phorbol myristateacetate and lonomycin promotes cell cycling in the CD2^–subset of CD4^–CD8^– T cells, and results in a slightinduction of CD2 levels during the course of the culture period.However, the majority of cells obtained at the end of the cultureperiod remain TCRß+ CD4^–CD8^–, CD2^low/–and B220^high, similar to freshly isolated CD4^–CD8^–Ipr T cells. In contrast, if IL-2 is included in the cultures,a strong shift toward a CD2⁺ phenotype is observed by a majorityof the Ipr T cells. Upon repeat stimulation, these Ipr CD4^–CD8^–T cells can now proliferate in an IL-2-dependent manner whenstimulated with only anti-CD3 mAb or mitogens, in the absenceof exogenous IL-2 or anti-CD28 mAb. These data show that thehyporesponsiveness of Ipr CD4^–CD8^– T cells doesnot result from a lack of CD28 expression, that it is not afixed state, and that it can be reversed by the induction ofcell cycling in the presence of IL-2. These observations extendthe parallels between Ipr CD4^–CD8^– T cells and anergicT cells.     

Selection of intestinal intraepithelial lymphocyte T cell receptors: evidence for a dynamic tissue-specific process

An extensive comparison of TCRß V-reglon usage byCD8ß^-CD4⁺CD8⁺ intraepithelial lymphocytes (IEL), CD4^-CD8⁺IEL, and lymph node (LN) T cell subsets in three minor lymphocytestimulating (MIs)-disparate, MHC-ldentical mouse strains revealednovel TCR selection patterns. In cases where forbidden V regionswere expressed by CD8ß^- CD4^-CD8⁺ IEL, the same TCRswere deleted from CD8ß^– CD4⁺CD8⁺ IEL, Indicatingthat lack of CD8ß expression was not solely responsiblefor forbidden V-region expression. These results also suggestedthat CD4 may be involved in negative selection of CD4⁺CD8⁺ IELTCRs. In C57BR/cdJ (Mls-1^b2^b) mice, a major increase in V_ß3⁺CD4⁺CD8⁺IEL but not in other IEL or LN subsets was noted suggestinga subset-specific expansion of V_ß3⁺ cells. Negativeselection of V_ß14⁺ cells in only the CD4⁺CD8⁺ IELsubset further supported the existence of intestine-specificTCR selection processes. Analysis of V-reglon expression ofCD8ß⁺ and CD8ß^–CD4^–CD8⁺ IELsubsets revealed that forbidden V-region expression was notstrictly confined to the CD8ß^– subset in allcases. Overall, the data point to a dynamic, gut-specific TCRselection process that may be antigen driven.     

Fas (CD95)-independent regulation of immune responses by antigen-specific CD4 CD8+ T cells

Antigen-activated T cells of the CD4⁺CD8^– and the CD4^–CD8⁺phenotype are susceptible to antigen receptor-stimulated celldeath. This form of apoptotic cell death has been shown to bedependent on the expression of the Fas (CD95) antigen and canoccur via an autocrine mechanism involving the concomitant up-regulationof Fas and its ligand on activated T cells. Mutations in genesencoding Fas (lpr) and the Fas ligand (gld) contribute to thedevelopment of an autoimmune syndrome similar to systemic lupuserythematosus in mice. These observations led to the suggestionthat the Fas signaling pathway is an important regulator ofimmune responses in vivo. Here we evaluated the importance ofthe Fas pathway in regulating immune responses by male antigen-specificCD4^–CD8⁺ T cells. We found that the in vivo eliminationof male antigen-activated cells was independent of Fas expressionby these cells. However, the elimination of these activatedcells was inhibited by the transgenic expression of Bcl-2, aprotein that inhibits multiple forms of apoptotic cell death.The transgenic Bcl-2 protein also inhibited the death of maleantigen-activated cells following IL-2 deprivation. Cell deathresulting from IL-2 deprivation occurred efficiently in maleantigen-activated Fas^- cells. We propose that the rapid deletionof male antigen-activated Fas^– cells in vivo is due tolimiting amounts of IL-2 that are available in the microenvironmentof the activated cells at the peak of the response.     

CD4+ICOS+ T lymphocytes inhibit T cell activation 'in vitro' and attenuate autoimmune encephalitis 'in vivo'

The inducible co-stimulator (ICOS, CD278) is essential to theefficient development of normal and pathological immune reactions.Since ICOS-deficient mice have enhanced susceptibility to experimentalallergic encephalomyelitis (EAE), we have functionally analyzeda CD4⁺ICOS⁺ population comprising 6–15% of all CD4⁺ Tcells in secondary lymphoid organs of unmanipulated wild-typemice and checked for their ability to suppress EAE. In C57BL/6mice, CD4⁺ICOS⁺ cells were a major source of cytokines includingIFN-, IL-2, IL-4, IL-10 or IL-17A. Upon activation, these cellsshowed preferentially enhanced production of IL-4 or IL-10 butinhibited IFN- production. In contrast, CD4⁺ICOS^– cellsmainly produced IFN-. Interestingly, CD4⁺ICOS⁺ cells partiallysuppressed the proliferation of CD4⁺ICOS^– or CD4⁺CD25^–lymphocytes ‘in vitro’ by an IL-10-dependent mechanism.Furthermore, CD4⁺ICOS⁺ activated and expanded under appropriateconditions yielded a population enriched in cells producingIL-10 and T_h2 cytokines that also suppressed the proliferationof CD4⁺CD25^– lymphocytes. CD4⁺ICOS⁺ cells, before or afterexpansion in vitro, reduced the severity of EAE when transferredto ICOS-deficient mice. In the same EAE model, lymph node cellsfrom ICOS-deficient mice receiving ICOS⁺ cells showed reducedIL-17A production and enhanced IL-10 secretion upon antigenactivation in vitro. Thus, naturally occurring CD4⁺ICOS⁺ cells,expanded or not in vitro, are functionally relevant cells ableof protecting ICOS-deficient mice from severe EAE.     

Correlation of effector function with phenotype and cell division after in vitro differentiation of naive MART-1-specific CD8+ T cells

Adoptive transfer of antigen-specific CD8⁺ T cells may representan effective strategy for immunotherapy of tumors such as melanoma,but is limited by the number and functionality of in vitro expandedT cells. Here, we document that although ELAGIGILTV-specificCD8⁺ T cells from different donors initially possessed a naivephenotype, after antigen-induced in vitro expansion two distinctphenotypes correlating with cell proliferation rate emergedin the different donors. Those cultures achieving fewer cumulativepopulation doublings (CPDs) were cytotoxic and displayed a CD45RA⁺CCR7^–phenotype. In contrast, cultures reaching higher CPDs were non-cytotoxicT cells with a CD45RA^–CCR7^– phenotype. Thus, thegeneration of larger numbers of ELAGIGILTV-specific CD8⁺ T cellscorrelates negatively with the acquisition of a CD45RA⁺CCR7^–phenotype and cytotoxic capacity. A better understanding ofthe differentiation pathways of cytotoxic T cells to obtainoptimally efficient cells for adoptive transfer will allow thedevelopment of new immunotherapy protocols.     

T cell receptor-triggered activation of intraepithelial lymphocytes in vitro

Intraepithelial lymphocytes (IEL) of the mouse small intestinewere examined for their potential to respond to TCR signallingin vitro. Purified IEL subsets were activated using mAbs specificfor CD3, TCRßor TCR&. Thy-1⁺IEL, regardless ofTCR type, proliferated equally well in response to anti-TCRmAb with or without exogenous IL-2. In contrast, Thy-1^–TCR, CD8^– IEL required exogenous IL-2 for proliferation.No such requirement was observed for Thy-1^– TCR& IELproliferation. IEL proliferation in the absence of added IL-2was due to an IL-2 secretion/IL-2 receptor (IL-2R) autocrinepathway, since mAbs specific for IL-2 and IL-2R inhibited IELproliferation. Thy-1⁺ CD8ß^– CD4⁺CD8⁺ IEL wereunresponsive to TCR-induced proliferation but exhibited highlevels of cytolytic activity upon TCR-triggerlng. Thy-1^–non-cytolytic IEL were induced to express Thy-1 and cytolytlcactivity following activation in vitro. In addition, the involvementof the co-stimulatory molecule CD28 in IEL activation was tested.CD28 was weakly expressed by fresh IEL and anti-CD28 mAb hadno effect on TCR-triggered proliferation. However, anti-TCRstimulation increased CD28 expression on a subset of TCRßIEL and the addition of anti-CD28 mAb resulted in increasedIL-2 production, but not in increased proliferation. Our resultsindicate that IEL, including the purported extrathymlc CD8ß^–subset, can respond to TCR-driven signals via proliferationand/or cytolytlc activity.     

ER-MP12 antigen, a new cell surface marker on mouse bone marrow cells with thymus-repopulating ability: II. Thymus-homing ability and phenotypic characterization of ER-MP12-positive bone marrow cells

In the accompanying paper we showed that six distinct subsetsof bone marrow (BM) cells can be identified using the mAb ER-MP12and ER-MP20 in two-colour immunofluorescence analysis. Uponintrathymic transfer into sublethally irradiated mice thymus-repopulatingability was restricted to ER-MP20^– BM cells expressingeither high or intermediate levels of the ER-MP12 antigen (1–2%and –30% of BM nucleated cells respectively). The highestfrequency of thymus-repopulating cells was found in the minorsubset of ER-MP12⁺⁺20^– BM cells. In the present studywe demonstrate that upon intravenous transfer, thymus-homingand-repopulating BM cells are exclusively confined to the ER-MP12⁺⁺20^–and ER-MP12⁺20^– subpopulations, the highest frequencybeing detected among ER-MP12⁺⁺20^– BM cells. Analysis ofthe peripheral blood leucocytes of reconstituted mice showedthat not only prothymocytes but also progenitorcells of theB cell lineage as well as the myelold lineage were present withinboth subsets. Three-colour flow cytometric analysis revealedthat ER-MP12⁺⁺20^– BM cells in particular were phenotyplcallyheterogeneous with respect to the expression of the cell surfacemarkers Thy-1, Sca-1, CD44, B220 and c-kit. Taken together ourdata demonstrate that ER-MP12 positively identifies BM cellswith the ability to home to and repopulate the thymus. The phenotypicheterogeneity displayed by the ER-MP12⁺⁺20^– BM subset,containing the highest frequency of thymus-homing and-repopulatingcells, provides a basis for further separation of prothymocyteactivity from other haematopoietic activities in the BM of themouse.     

Induction of phosphocholine-specific antibodies in X-linked immune deficient mice: in vivo protection against a Streptococcus pneumoniae challenge

X-linked Immune deficient (XID) mice are susceptible to infectionwith Streptococcus pneumoniae because they fail to mount animmune response to the Immunodomlnant phosphochollne (PC) epltopeon the bacterial cell wall. It is difficult to induce PC-speclflcantibodies in XID mice because PC-specific B cells expressingthe T15-, M167- and M603 Idiotype (Id), which provide protectionagainst S. pneumoniae, are deleted in these mice via an antigen-specific,receptor-mediated process. In addition, the standard PC hapten,p-dlazophenylphosphochollne (DPPC), induces high affinity phenylphosphochollne(PPC)-speciflc antibodies in XID mice, which are not protectiveagainst S. pneumoniae. We have used a novel PC hapten, p-nitrophenyl-6-(O-phospho-choline)hydroxyhexanoate(EPC), to induce PC-specific antibodies in XID mice. The immuneresponse to EPC-keyhole limpet hemacyanln (KLH) is dominatedby IgGi, V_H1⁺, US-Id^–, PC-inhlbitable antibodies. A smallIgM antl-PC response having a consistent T15-ld⁺ component isalso induced in XID mice, whereas normal mice produce a largeIgM response dominated by T15-ld⁺ antibodies. The immune responseto EPC-KLH remains predominantly PC-lnhlbltable even after multipleimmunizations, while the response to DPPC–KLH becomesdominated by PPC-speclflc antibodies. C.CBA/N mice immunizedtwice with EPC–KLH are protected against 10⁴ S. pneumoniaewhile as few as 10 bacteria are 100% lethal for the unlmmunlzedcontrols. The ability of EPC-protein to induce a long-lived,PC-speclflc response should make this hapten a potential TDvaccine candidate for S. pneumoniae     

Endocrinology: The effect of high-dose medium- and long- term progestogen exposure on endometrial vessels

A total of 19 paraffin-embedded endometrial tissue blocks wereobtained from high-dose progestogen-exposed patients. A labelledstreptavidin—biotin—alkaline phosphatase methodwas used with antibodies against von Willebrand factor (vWF)and CD34. The density of CD34 and vWF positive (CD34⁺ and vWF⁺)vessels in progestogen-exposed endometria (103 ± 9.6/mm²and 106 ± 8.7/mm²) was significantly lower than in endometriafrom women with normal cycles (169 ± 9.3/mm² and 136± 8.0/mm²) (P < 0.05). In women with normal menstrualcycles the concentration of CD34⁺ vessels was significantlyhigher than the number of vWF⁺ vessels (P = 0.0001). By comparison,the concentration of CD34⁺ vessels was similar to the concentrationof vWF⁺ vessels in progestogen-exposed endometria. The ratiosof vascular density as determined by vWF⁺ and CD34⁺ stainingin the control and progestogen groups were 0.81 and 1.05 respectively(P = 0.0001). Dilated venules were seen in the progestogen group.This study has demonstrated firstly that CD34 antibody detectedthe endothelial cells in a higher proportion of small endometrialvessels than vWF, and secondly that high-dose progestogen exposuresignificantly decreased the density of microvessels and increasedthe number of dilated venules in endometrium.     

Human CD4/CD45RA+ and CD4/CD45RA T cell subsets express CD4-p56lck complexes, CD4-associated lipid kinases, TCR/CD3-p59fyn complexes, and share similar tyrosine kinase substrates

T cell activation appears to be regulated by an interplay betweenprotein tyrosine kinases (PTKs) and protein tyrosine phosphatases(PTPases). p56^lck and p59^fyn have been found to associate withCD4 and TCR-CD3 respectively. The CD45 family of transmembranePTPases has been shown to be able to regulate the activitiesof these receptor-associated PTKs in vitro. In man, CD45 containsfive different isoforms whose distribution defines subsets ofT cells having distinct activation requirements and in vitrofunctions.Several groups have reported a physical interaction betweendistinct isoforms of CD45 and CD2, CD4, and the TCR-CD3 complex.Given the potential regulatory interaction between CD45 andPTKs in CD4⁺ subsets expressing different CD45 isoforms, wehave examined CD4 associated and TCR-CD3^– associated PTKactivities, associated phosphatidyl inositoi (PI) kinases andsubstrates of tyrosine phosphoryiation in CD45RA⁺and CD45RA^–CD4⁺ T cell lines derived from peripheral blood. Both subsetsexpress CD4-assoclated p56^lck and TCR-CD3-associated p59^fynkinases which exhibit identical in vitro phosphoryiation atthe Y-394 and Y-420 autophosphorylation sites respectively.Further, both subsets exhibited PI kinases activity associatedwith CD4-p56^lck. Consistent with these observations, anti-CD3crosslinklng induced the phosphoryiation of a similar spectrumof intracellular substrates in these CD45RA⁺and CD45RA^–CD4⁺ T cell lines. These observations indicate that despitethe possible interaction between CD45 isoforms and CD4 or TCR-CD3,the mere expression of the CD45RA isoform does not in and ofitself alter the presence of receptorassociated kinases or theirintracellular targets.     

Critical role of dendritic cells in determining the Th1/Th2 balance upon Leishmania major infection

The onset of T_h1 immunity is in part regulated by genetic background.To elucidate the cell type carrying critical factors determiningthe T_h1 response, we employed Rag-2^–/– mice on Leishmaniamajor-susceptible BALB/c and -resistant B10.D2 backgrounds.By using bone marrow (BM) chimeras generated by the transplantationof B10.D2 BM cells into BALB/c-Rag-2^–/– mice, andvice versa, it was shown that hematopoietic cells carry factorsdetermining the disease outcome and T_h1 response against L.major infection. B10.D2-Rag-2^–/– mice reconstitutedwith BALB/c CD4⁺ T cells exhibited a T_h1 response and controlledL. major infection. Wild-type BALB/c mice inoculated with L.major-parasitized B10.D2-Rag-2^–/– splenocytes alsoexhibited a T_h1 response and a mild disease outcome, whereassuch a T_h1 response was not induced when CD11c⁺ dendritic cells(DCs) were depleted from parasitized B10.D2-Rag-2^–/–splenocytes. T_h1 response was reconstituted by the additionof L. major-parasitized B10.D2 DCs but not L. major-parasitizedBALB/c DCs to DC-depleted parasitized B10.D2-Rag-2^–/–splenocytes. These results indicate that DCs determine the outcomeof the disease upon L. major infection.     

Superantigen-reactive T cells that display an anergic phenotype in vitro appear functional in vivo

Clonal deletion and/or inactivation establishes tolerance toself antigens. Endogenous and exogenous (bacterial) superantigens,like the staphylococcal enterotoxlns, induce ligand-specificclonal anergy in vivo and thus are believed to mirror aspectsof post-thymic tolerance mechanisms in mature peripheral T cells.Here we analyzed the level of anergy of ligand-responsive V_ß8⁺T cells from staphylococcal enterotoxin B (SEB)-primed micein vivo and in vitro. Upon in vitro restimulation with SEB,CD4⁺V_ß8⁺ and CD8⁺V_ß8⁺ T cells failed toproduce IL-2. However, functional IL-2 receptors were triggered,since supplementation with IL-2 induced clonal growth in virtuallyall CD4⁺V_ß8⁺ and CD8⁺V_ß8⁺ T cells as determinedby limiting dilution analyses. Thus in vitro unresponslvenessof lymphocytes from SEB-primed mice reflects the inability ofSEB-reactlve V_ß8⁺ T cells to produce IL-2. Surprisingly,anergy as defined in vitro was at variance with that in vivo.Following further challenge with SEB, systemic and acute lymphokineproduction (Including IL-2 and tumor necrosis factor) occurredwith almost identical peak values and kinetics to primary invivo responses, and D-galactosamlne-sensltlzed mice succumbedto lethal shock. Polymerase chain reaction analyses revealedthat CD4⁺V_ß8⁺ expressed IL-2-specific mRNA in vivoupon restimulatlon with SEB. While lymphokine production andexpression of the IL-2 receptor was similar to the responseto in vivo primary stimulation, only CD8⁺V_ß8⁺ T cellsexpanded clonally upon reintroductlon of SEB in vivo. Henceprimed V_ß8⁺ T cells challenged with SEB display invitro anergy yet in vivo responsiveness, at least in part. Weconclude that the state of anergy is reversible, dependent uponthe quality of activation signals provided in in vivo ratherthan in in vitro culture conditions.     

Natural killer cell activity in lymphocytic choriomeningitis virus-infected {beta}2-microglobulin-deficient mice

We have investigated the induction and role of natural killer(NK) activity in lymphocytic choriomeningitis virus (LCMV)-infectedß₂-microglobulin-deficient (ß₂m^–)mice. We demonstrate that LCMV infection is more effective thanpolyinosinic:poiycytidylic acid (poly I:C) at stimulating NKactivity in ß₂m^– .In addition, ß₂m^–NK cells respond poorly to in vitro treatment with IL-12. Thetarget specificity of the virally induced NK cells is similarto that previously reported for chemically induced ß₂m^–NK cells. In both cases they can lyse YAC-1 tumor cells butare unable to kill ß₂m^– orß₂m⁺ T cellblasts. We have also found that the time course of inductionof NK and cytotoxic T lymphocyte (CTL) activity by LCMV in ß₂m^–mice is delayed compared with normal mice. Maximal NK and CTLactivity is attained at day 8 and 10 post-infection respectivelyin ß₂m^– compared with day 4 and 6—8 inB6 mice. Whereas normal mice die {small tilde}7 days followingintracranial infection with LCMV, the course of disease in ß₂^–mmice is protracted and characterized by a marked loss of bodyweight. We show that although the CD4⁺ CTL response in thesemice is intimately involved in mediating weight loss, the virus-inducedNK cells do not appear to play a role in the disease.     

Proliferation and apoptosis of B220+ CD4 CD8 TCR{alpha}{beta}intermediate T cells in the liver of normal adult mice: implication for Ipr pathogenesis

Small numbers of T cells have been isolated from the normalmouse liver and many of these are of the CD4^–CD8^–TCRß⁺phenotype. Larger numbers of such cells are present in the liversof mice homozygous for the Ipr mutation and the liver has beenproposed to be the site of an extrathymlc T cell developmentpathway that is expanded in Ipr/lpr mice. Using a modified separationprocedure that increases the liver T cell yield, we have beenable to characterize a subset of CD4^–CD8^–TCRß^intermediateT cells that express the B220 epltope of the CD45 molecule,and resemble in this and many other ways the accumulating Tcells in Ipr lymph nodes. These cells are an actively dividingpopulation and even in healthy, unmanipulated mice a large proportionof them are undergoing apoptosis. We propose the model thatthe normal liver is a major site for T cell destruction andthat the Ipr defect results in failure of this process withleakage of B220⁺CD4^–CD8^–TCRß⁺ cells fromthe liver to peripheral lymphoid tissues, particularly lymphnodes.