CD4+CD45RA+T cells from adults respond to recall antigens after CD28 ligation

The leukocyte common antigen isoforms CD45RA and CD45RO havelong been used to discriminate human naive and memory T cellsrespectively. This model was largely based on the observationthat CD45RO⁺ T cells respond preferentially to and show a higherfrequency of precursors specific for recall antigens. However,CD45RA⁺ T cells have more stringent requirements for stimulationand standard in vitro assays may favour CD45RO⁺ cells in thisrespect. We tested the hypothesis that CD45RAf T cells respondpoorly to in vitro stimulation with recall antigens becauseof inadequate stimulation rather than a lack of precursors.Limiting dilution analyses (LDA) for tetanus toxoid (lT)-specificT cells were performed in the presence or absence of exogenousantLCD28 antibody. Addition of antLCD28 yielded no proliferationin the absence of specific antigen. The precursor frequencyfor lT in the CD4⁺ CD45RO⁺ population was –1:4000, whilethe frequency of CD4⁺ CD45RA⁺ T cells specific for lT was 4-to >>20-fold lower. Addition of anti-CD28 antibody didnot significantly alter the apparent precursor frequency forCD45RA⁺ cells but yielded an enhancement of the value for CD45RA⁺cells by 3- to >>5-fold. No enhancement of antigen-specificproliferation by antLCD28 was observed with CD45RA⁺ T cellsderived from cord blood, although phytohemagglutinin responsesof these cells were amplified by CD28 antibody. These resultsindicate that conventional LDA underestimate the true precursorfrequency of antigen-specific cells within the adult CD45RA⁺population and support the possibility that a small number ofcells revert from a primed (CD45RO⁺) to an unprimed (CD45RA⁺)state. The majority of memory T cells, however, appear to residein the CD45RO⁺ population     

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.     

Differential effect of transforming growth factor-{beta}1 on the activation of human naive and memory CD4+ T lymphocytes

Transforming growth factor-ß1 (TGF-ß1) canhave stimulatory or inhibitory effects on cell growth. For severalcell types, the effect of TGF-ß1 was found to correlatewith the differentiation stage of the cells and the presenceof other cytoklnes. We have studied here the influence of TGF-ß1on CD4⁺ T cell activation in relation to the differentiationstage of the cells by evaluating the effect of TGF-ß1on the prollferatlve responses of purified CD4⁺CD45RA⁺ (unprfmed)and CD4⁺CD45RO⁺ (primed) lymphocytes. Under certain conditions,TGF-ß1 exerted a co-stlmulatory effect on peripheralblood CD4⁺CD45RA⁺ T cells whereas the outgrowth of CD4⁺CD45RO⁺T cells was suppressed in any activation system tested. Theenhancement of prollferatlve responses by TGF-ß1 inTCR/CD3 or CD2 stimulated cultures of CD45RA⁺ cells involvedup-regulatlon of CD25 expression and was dependent on the presenceof exogenous IL-2 or CD28 mAbs; IL-7 driven proliferatlve responseswere suppressed by TGF-ß1. These observations wereconfirmed in experiments with purified cord blood (CB) CD4⁺T cells inasmuch as addition of TGF-ß1 caused a 2-to 7-fold increase in IL-2 driven proliferatlve responses ofthese cells. Finally we show that, in contrast to the effectof TGF-ß1 during primary stimulation of CB CD4⁺ Tcells, TGF-ß1 suppressed T cell proliferation for40% in secondary cultures of these cell. Our findings indicatethat TGF-ß1 Is a blfunctlonal regulator of CD4⁺ Tcell growth in vitro, with co-stimulatory capacities duringCD45RA⁺ T cell mediated primary responses and growth suppresslveeffects during secondary responses of CD45RO⁺ T cells.     

Role of CD4+CD45RA+ T cells in the development of autoimmune diabetes in the non-obese diabetic (NOD) mouse

The non-obese diabetic (NOD) mouse spontaneously develops aT cell-mediated autoimmune disease, sharing many features withhuman insulin-dependent diabetes mellltus (IDDM), leading toinsulin-secreting ß cell destruction. The role ofCD4⁺ T cells has been evidenced at two levels. First, CD4⁺ Tcells from diabetic animals are required to transfer diabetesto non-diabetic recipients in conjunction with CD8⁺ effectorT cells. Second, suppressive CD4⁺ T cells have been characterizedin non-diabetic NOD mice. T cells with different functions canthus share the CD4⁺ phenotype. Since CD4⁺ T cells can be dividedinto at least two subgroups on the basis of CD45 isoform expression,we evaluated the distribution of CD4⁺ T cells expressing theCD45RA isoform on NOD mouse thymocytes and peripheral T cells.The percentage of CD45RA⁺ cells was dramatically increased amongthe most mature CD3^bright thymocytes and among CD4⁺ T cellsin lymph nodes of the NOD mouse as compared with control strains.This increase was related to the development of insulitls. Interestingly,the CD45RA isoform was expressed on most CD4⁺ T cells invadingthe islets. In vivo treatment with an antl-CD45RA mAb preventedthe development of insulitls and spontaneous diabetes in femaleanimals but not the transfer of diabetes by T cells collectedfrom diabetic NOD donors. These results indicate that anti-CD45RAmAb is only effective if given before the full commitment ofeffector T cells to the destruction of islet ß cells.ThusCD4⁺CD45RA⁺ T cells play a key role in early activation stepsof anti-islet immunity.     

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.     

Phenotypically distinct subsets of CD4+ T cells induce or protect from chronic intestinal inflammation in C. B-17 scid mice

CD4⁺ T cells in the mouse can be subdivided into two fractionsbased on the level of expression of the CD45RB determinant.Previous studies have shown that these subsets are functionallydistinct. We have further characterized the properties of thesesubpopulations in vivo by injecting them into C. B-17 scid mice.The animals restored with the CD45RB^highCD4⁺ T cell populationdeveloped a lethal wasting disease with severe mononuclear cellinfiltrates into the colon and elevated levels of IFN- mRNA.In contrast, animals restored with the reciprocal CD45RB^lowsubset or with unfractionated CD4⁺ T cells did not develop thewasting or colitis. Importantly, the co-transfer of the CD45RB^lowpopulation with the CD45RB^high population prevented the wastingdisease and colitis. These data indicate that important regulatoryinteractions occur between the CD45RB^high and CD45RB^lowCD4⁺T cell subsets and that disruption of this mechanism has fatalconsequences.     

CD45RO+ memory T cells but not CD45RA+ naive T cells can be efficiently activated by remote co-stimulation with B7

Co-stimulatory signals are absolutely required for T cell activationafter TCR–MHC-peptide interaction. The most importantco-stimulatory signal known so far is mediated by the interactionof CD28 on T cells with B7 on APC. Here we demonstrate thatthe co-stimulatory signal from the B7 molecule does not necessarilyhave to come from the same cell which presents antigen. Titrationcurves obtained by limiting the amount of anti-CD3 mAb suggeststhat the same amount of TCR–CD3 cross-linking is requiredfor full T cell activation whether B7 is present on the sameor on another cell, but that the kinetics of T cell activationis slower when B7 is present on a separate cell from the primarysignal. Finally and most importantly we also show that CD45RO⁺memory T cells, but not CD45RA⁺ naive T cells, can be efficientlyactivated when B7 is expressed on bystander cells. These findingsimply that co-stimulatory activation requirements of B7 aremore stringent for naive than for memory T cells, which couldbe an important mechanism involved in the maintenance of self-tolerance.     

Functional CD4+T cell subsets defined by expression of CD45RC and NTA260 antigens and age-associated polarization in murine lupus

Using two mAb, one specific to the alternative exon 6-dependentepitope of CD45 molecules(JH6.2) and one a natural thymocytotoxicautoantibody (NTA) with an unknown reactive epitope (NTA260),we subdivided splenic CD4⁺ T cells from 2-month-old BALB/c miceinto five phenotypically distinct subsets. CD45RC⁺NTA260^–(SI) cells were phenotypically analogous to CD4⁺ T cells predominatingin newborn mice and produced a significant amount of IL-2, butnot so IL-4, IL-10 or IFN- when stimulated with immobilizedanti-CD3 mAb in vitro. They appeared to consist mainly of naiveT_hP cells. The CD45RC⁺;NTA260⁺ (S II) subset also produced IL-2,but not other cytokines; however, the IL-2 levels produced weremuch higher than seen with the S I subset, thereby suggestingthe predominance of further maturated T_hP cells. The D45RC^–NTA260⁺(S III) subset mainly produced IL-4, IL-10, IFN- and less IL-2,and contained memory cells that helped the secondary antibodyresponse to a recall antigen, and hence contained T_h2 and probablya mixture of T_h0 and T_h1 cells. The CD45RC^–NTA260^–(S IV) subset was a poor responder to the immobilized anti-CD3mAb. The CD45RC^brightNTA260^dull(S V) subset consisted of a smallnumber of cells that were phenotypically analogous to activatedCD4⁺ T cells. While an age-associated decrease in the proportionof S I and less markedly in S II and in turn increase in S IIIsubsets of CD4⁺ T cells occurred in normal BALB/c mice, autoimmunedisease-prone (NZBxNZW)F₁ mice showed a marked age-associateddecrease in the proportion of not only S I, II but also IIIsubsets. As aged (NZBxNZW)F₁ mice carry CD4⁺ T helper cellsfor IgG anti-DNA antibody production, such age-associated polarizationto the S IV subset appears to be critical in the pathogeneslsof autoimmune disease in these mice.     

Changes in the subsets of CD4 + T cells in Trypanosoma musculi infection: delay of immunological cure in young mice and the weak ability of aged mice to control the infection

After a 3 week course (approximately), during which there ismarked lymphoid hyperplasia, Trypanosoma musculi infectionsin young-adult mice are cured by an immune mechanism involvingantibodies of the IgG2a isotype. Both the lymphoid hyperplasiaand IgG2a antibody response are T-cell-dependent events andboth processes appear to be defective in aged mice. The purposeof the studies reported here was to elucidate the effects ofT.musculi infection on subsets of T cells for two reasons: (I)to gain insight into the probable roles of selected cytokines(IL-2, IL-4 and IFN-) in facilitating the production of curative,lgG2a antibodies, and (II) to examine the hypothesis that agingaffects the competence of CD4⁺ T cells to participate in immunologicalcontrol of infections. The major conclusions from these studiesare that: (I) T. musculi infection of mice induces rapid changein the CD4⁺ T cell population toward predominance of the activatedor memory (CD45RB^loCD44^hi) phenotype, cells which produce IFN-,II-3. IL-4 and IL-5, accompanied by profound Inhibition of IL-2production, and (II) in the old mice these changes are superimposedon the natural age-associated changes in the same direction(i.e. toward predominance of CD45RB^loCD44^hi T cells).Thus, inthe old animals, the combined changes of aging and infection,moving in the same direction, are devastating, resulting inthe aged animals being unable, or barely able, to control infection.     

Activation of CD45-deficient T cell clones by lectin mitogens but not anti-Thy-1

CD45, the leukocyte-common antigen, Is a transmembrane proteintyrosine phosphatase uniquely expressed by cells of hematopoletlcorigin. We have developed CD4⁺ and CD8⁺ T cell clones that aredeficient in the expression of CD45 and have previously shownthat these cells fall to proliferate in response to antigenor cross-linked CD3. These studies have now been extended toshow that stimulation with antl-Thy-1, a mltogenlc signal forthe CD4⁺CD45⁺ and CD8⁺CD45⁺ T cells, falls to induce proliferationin the CD45^– T cells. Examination of the CD8⁺CD45^–T cells correlates antl-Thy-1 unresponslveness with a failureto increase in tyrosine phosphorylatlon. Furthermore, stimulationof CD8⁺CD45⁺ T cells with antl-Thy-1 results in an increasein p56^ick activity but not in CD8⁺CD45^– T cells. In contrastto the results with antl-Thy-1, both the CD4^+– CD45 andCD8⁺CD45^– T cells respond to treatment with lectin mitogens,concanavalln A or phytohemagglutlnln. Lectin-lnduced proliferationwas inhibited by the addition of cyclosporln A. Treatment ofCD45^– T cells with PMA and lonomycln also results in proliferationindicating that activation of protein kinase C in conjunctionwith an increase in intracellular calcium rescues the defectcafsed by CD45 deficiency. The data suggest that CD45 Is requiredfor the activation of tyrosine kinase activity Immediate orprior to transmembrane signaling.     

In vivo induction of CD4+ T cell responses by antigens covalently linked to synthetic microspheres does not require adjuvant

Macrophages, dendritic cells or B lymphocytes have been shownto play a major role in the presentation of soluble antigensto CD4⁺ T cells. In contrast, the capacity of these cells topresent particulate antigens such as bacterial or parasiticantigens to T cells remains controversial. To investigate thisquestion, well defined particulate antigens were prepared bycovalent linkage of proteins or peptides to 1 µm in diametersynthetic microspheres. The T cell immunogenicity of such particulateantigens was analyzed in vitro and in vivo. In vitro, a solubleprotein such as hen egg lysozyme (HEL) coupled to beads stimulateda strong proliferative T cell response of lymph node cells fromHEL-primed mice or of specific T cell hybridomas. HEL coupledto beads was presented to the specific T cell hybridomas bysplenocytes or by peritoneal macrophages, but not by lymphomaB cells. Immunization of mice with several different proteinantigens or with a synthetic peptide covalently linked to beadsinduced strong CD4⁺ T cell responses in the absence of adjuvant.The strong in vivo immunogenicity of proteins coupled to beadsdid not result from a non-specific adjuvant effect of beadssince covalent linkage of the antigen to beads was strictlyrequired to induce T cell responses in the absence of adjuvant.In vivo treatment by carrageenan showed that macrophages arerequired for the in vivo stimulation of T cell responses bythese particulate antigens. Thus, these results demonstratedthe role of phagocytic cells, especially macrophages, for invivo presentation of particulate antigens. These particulateantigens represent an interesting approach for the developmentof new vaccines, and for the in vivo analysis of the role ofvarious antigen presenting cells in T cell activation and differentiation.     

CD28 ligands CD80 (B7-1) and CD86 (B7-2) induce long-term autocrine growth of CD4+ T cells and induce similar patterns of cytokine secretion in vitro

The interaction of CD28 and its ligands is critical for antigen-inducedT cell activation. Recent studies have demonstrated the existenceof at least two members of the B7 receptor family. In this report,the co-stimulatory signals provided by CD80 (B7-1) or CD86 (B7-2)were compared to CD28 ligation by mAb. We demonstrate that thekinetics of induction of T cell proliferation after anti-CD3stimulation was similar regardless of the form of co-stimulation.Similarly, B7-1 and B7-2 could both maintain long-term expansionof CD4 cells. The co-stimulatory effects of both B7-1 and B7-2were dependent on CD28 cross-linking, based on complete inhibitionof proliferation by CD28 antibody Fab fragments. Co-stimulationwith B7-1 and B7-2 induced high levels of cytokine secretionby resting T cells, and the effects of B7-1 and B7-2 could notbe distinguished. This conclusion is based on analysis of theinitial activation of CD28⁺ T cells. as well as T cell subpopulationsconsisting of CD4⁺ and CD8⁺ T cells. Both B7-1 and B7-2 couldelicit IL-4 secretion from CD4⁺ T cells while anti-CD28 antibodyinduced substantially less IL-4 secretion. Furthermore, bothB7-1 and B7-2 could stimulate high levels of IFN- and IL-4 fromCD4⁺CD45RO⁺ cells, while neither B7 receptor could co-stimulateIFN- and IL-4 secretion from CD4⁺CD45RA⁺ T cells. B7-1 and B7-2could, however, co-stimulate CD4⁺CD45RA⁺ T cells to secreteIL-2. By contrast, when previously activated T cells were tested,re-stimulation of CD4⁺ T cell blasts with B7-1 or B7-2 resultedin higher secretion of IL-4 and IL-5 than anti-CD28, while re-stimulationwith anti-CD28 antibody maintained a higher level of secretionof IL-2 and IFN- than B7-1 or B7-2. These observations may haveimportant implications because they suggest that the mannerof CD28 ligation can be a critical determinant in the developmentof cytokine secretion that corresponds to T_h1- and T_h2-likepatterns of differentiation. Together these observations suggestthat there are no Intrinsic differences between B7-1 and B7-2in their ability to co-stimulate the populations of cells thatwe have tested.     

Age-related accumulation of LFA-1high cells in a CD8+CD45RAhigh T cell population

The differential expression of CD45 isoforms has been suggested as a marker for stages of post-thymic T cell development, that is, CD45RA+CD45R0^? T cells and CD45RA^?CD45R0⁺ T cells are supposed to be virgin and memory cells respectively. Recently, several adhesion molecules have been shown to be up-regulated on the cell surface of memory T cells, and have been suggested to serve as a memory marker. In this study, we investigated the levels of LFA-1 expression on T cells in various subpopulations defined by CD45 isoform expression in donors of various ages. In CD4⁺ T cells, the proportion of LFA-1^high cells among CD45RA^highCD45R0-T cells remained low in all age groups and did not show significant accumulation with age. CD4⁺CD45RA^?CD45R0^highTcells expressed LFA-1 at a higher level than CD4⁺CD45RA^highCD45R0^? T cells. Thus, the currently prevailing view that CD45RA and CD45R0 can be markers for virgin and primed cells was consistent with LFA-1 expression in CD4⁺ T cell population. In CD8⁺ T cells, however, CD45RA^highCD45R0^? T cells consisted of two distinct subpopulations, LFA-1^low and LFA-1^high cells, whereas CD45RA^?CD45R0^high T cells were almost exclusively LFA-1^high When CD29 expression was examined in place of LFA-1 expression, similar results were obtained; CD45RA^high CD45R0^? T cells consisted of two distinct subpopulations, CD29-^{to low} and CD^29high cells, while CD45RA-CD45R0^high T cells were mostly CD29^high. The proportion of LFA-1^high cells in the CD8+CD45RA^high T cell subpopulation increased significantly as a function of age (r = 0.9, p < 0.001). It ranged from 8% in a 14-year-old donor to 94% in a 79-year-old donor. Furthermore, the proportion of CD8⁺CD45RA^highLFA-1^high cells in the CD8⁺ T cell population increased significantly as a function of age (r = 0.85, p < 0.001). On the other hand, the proportion of LEA-1^high cells in CD8⁺CD45RA^? T cell subpopulation exceeded 90% in most donors irrespective of age. These results indicate that the CD8⁺CD45RA^highCD45R0^? T cell subpopulation contains a considerable number of LFA-1^high cells and CD29^high cells, phenotypically similar to previously activated cells. Thus, in terms of LFA-1 and CD29 expressions, the simple scheme that CD45RA is a marker of virgin cells is not applicable to the CD8⁺ T cell population.     

Evidence for selective in vivo expansion of synovial tissue-infiltrating CD4+ CD45RO+ T lymphocytes on the basis of CDR3 diversity

In this study we have analyzed the TCR V and V_ß regionsat the DNA level in the CD4⁺CD45RO⁺ memory T cell populationof synovial tissue infiltrating T lymphocytes of three rheumatoidarthritis (RA) patients and one patient with chronic arthritis.Cell lines of CD4⁺CD45RO⁺, CD4⁺CD45RO^–, CD8⁺CD45RO⁺ andCD8⁺CD45RO^– T lymphocyte populations were generated followingFACS cell sorting of freshly isolated synovial tissue mononuclearcell infiltrates (STMC) and of freshly isolated peripheral bloodmononuclear cells (PBMC) of these patients. The phenotyplc andmolecular analyses have revealed the following. (I) The TCRrepertoires of tissue infiltrating T lymphocytes in the varioussubsets were extensive on the basis of TCR V gene family usage.(II) Furthermore, each patient displayed individual specificTCR V gene expression patterns in the various STMC and PBMCderived T cell subsets. However, the majority of these arthritispatients manifested increased expression of multiple TCR V genefamilies in the synovial tissue derived CD4⁺CD45RO^– Tcell population when compared with the peripheral blood derivedCD4⁺CD45RO⁺ subset. Of these gene families, we found enhancedexpression of the TCR V7 and V_ß11 gene segments insynovial tissue to be shared by all four patients analyzed.OH) Nucleotlde sequence analysis of the CDR3 regions of a numberof TCR V regions in the CD4⁺CD45RO⁺ T cell subsets has revealedthat the CDR3 regions comprised within synovial tissue derivedTCR V regions differed from those found in peripheral bloodderived TCR V regions. These differences in CDR3 diversity mightbe the consequence of a specific interaction with particularMHC-peptlde complexes expressed at the site of inflammation.(Iv) The CDR3 region analysis also showed individual specificamlno acid motifs within the N-D-N regions of all analyzed TCRV_ß genes derived from PBMC as well as STMC.     

Resident CD4+ {alpha}{beta} T cells of the murine female genital tract: a phenotypically distinct T cell lineage that rapidly proliferates in response to systemic T cell activation stimuli

A population of CD4⁺ cells has been identified in the murinefemale genital tract (FGT). Phenotypic studies of FGT CD4⁺ cellsdemonstrate that they express CD3 and that the majority of thesecells are ßTCR⁺Thy-1⁺. Most of the Thy-1⁺CD4⁺ßTCR⁺ cells resemble memory T cells based on their expressionof CD44, L-selectin and CD45RB antigens. The vast majority ofThy-1⁺CD4⁺ßTCR⁺ FGT cells are CD5⁺ and all of themare B220^–. Systemic stimuli including infection with Trypanosomabrucel brucel, injection with anti-CD3, or bacterial superantigensstaphylococcal enterotoxin A or B cause a rapid accumulationof CD4+cells in the FGT exceeding that observed for CD4⁺ cellsin spleen and lymph nodes (LN). Expansion of the FGT CD4⁺ cells,which are phenotypically distinct from the splenic and LN CD4⁺T cells, is due to local proliferation rather than an influxof cells from the circulation. The CD4⁺ population in the FGTof adult nu/nu mice is dramatically reduced, indicating itsthymic dependency. In lpr/lpr mice, FGT CD4 cells do not displaychanges characteristic of splenic or LN CD4 cells in the sameanimals. These findings demonstrate that the CD4⁺ cells of themurine FGT are thymic dependent, but that they constitute aT cell lineage that phenotypically and, probably functionally,is distinct from other peripheral CD4⁺ T cell populations.     

Activated T and B lymphocytes in peripheral blood of patients with Chagas' disease

Whole blood preparations from patients with either the indeterminate(asymptomatic) or cardiac clinical forms of chronic Trypanosomacruzl infection were analyzed by flow cytometry using double-labelingto identify subsets of circulating lymphocytes. Several significantdifferences were demonstrated between the blood lymphocyte profilesof chagaslc patients and non-chagaslc controls. Clear increasein the percentages and actual numbers of double-positive cellsof the phenotype CD3⁺/HLA-DR⁺, as well as decrease in the percentageof CD45RA⁺/CD4⁺ and CD45RA⁺/CD8⁺ T cells, Indicate greater numbersof activated T cells circulating in the blood of infected patients.Consistent parallel increases were seen also in the B lymphocytesubset which stained double-positive for CD19/CD5. There wereno significant differences in the circulation of these chronicchagaslc patients in the CD4:CD8 ratios. Also, no substantivephenotyplc differences were observed in the lymphocyte populationsbetween the two ends of the clinical spectrum (Indeterminateversus cardiac) in chronic human Chagas' disease. These observationsdemonstrate that increased levels of activated T cells and CD5⁺B cells are present in the circulation of people with chronicChagas' disease. These are cell phenotypes that have been associatedin other conditions with autoimmune, polyclonal, and hyperlmmuneresponses. The specificities of these activated cells and theroles they may play in resistance or pathogenesis during chronicChagas' disease need now to be determined.     

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.     

Alternative complement pathway activation by CD4+ T cells of HIV infected individuals: a possible role in AIDS pathogenesis

The mechanisms by which CD4⁺ T cells are eliminated during HIVinfection are poorly understood. We have previously shown thatHIV infected cell lines activate and fix C3 via the alternativecomplement pathway (ACP). In the present study we examined theability of blood lymphocytes from 40 HIV⁺ individuals to fixC3. A large fraction of the CD4⁺ T cells reacted with anti-gp120antibodies. These cells also carried C3 fragments in vivo andcould further fix C3 if exposed to human serum In vitro. C3activation occurred via the ACP. In some cases exposure of thelymphocytes to human serum under conditions allowing ACP activationresulted in partial elimination of CD4⁺ T cells. The resultssuggest that complement activation and fixation by CD4⁺ T cellsopsonized with HIV particles or gp120 may contribute to theirselective destruction.     

Antigen-independent adhesion of CD4 CD45RA T cells from cord blood

Antigen-independent adhesion of resting adult CD4⁺ CD45RO⁺ T cells to B lymphocytes has been shown to be transient and can be down-regulated by CD4 major histocompatibility complex (MHC) class II molecule interactions. Conversely, adhesion of adult CD4+ CD45RA+ subpopulation to B cells is not regulated by ligands of CD4. We have investigated the regulation of adhesion of cord blood CD45RA⁺ CD4⁺ T lymphocytes. In contrast to adult CD45RA⁺ CD4⁺ T cells, cord blood CD45RA⁺ CD4⁺ T cells were strongly sensitive to the down-regulation of adhesion mediated by the CD4-HLA class II interaction, since adhesion to MHC class II(⁺) B cells was transient and inhibited by an anti-CD4 antibody. In addition, human immunodeficiency virus gpl60, synthetic gpl06-derived peptides encompassing a CD4 binding site inhibited conjugate formation between cord blood CD45RA⁺ CD4+ T cells and B cells. Following activation of the cord blood CD4 T cells by an anti-CD3 antibody, a conversion from a transient to a stable adhesion pattern of cord blood CD4 T cells to B cells occurred in 2 days. The reversal to a transient adhesion occurred at day 8 following anti-CD3 activation in correlation with a complete shift to a CD45RO phenotype of the cord blood CD4 T cells. These data suggest that CD4 T cell adhesion can be developmentally regulated.     

Altered response of CD4+ T cell subsets to Plasmodium chabaudi chabaudi in B cell-deficient mice

Mice depleted of B cells from birth by treatment with anti-_µantibodies can control but not clear an infection with the malariaparasite Plasmodium chabaudi chabaudi (AS). Splenic CD4⁺ T cellsfrom these mice were unable to mount a significant T_h2 responseto the parasite in vitro as shown by much lower precursor frequenciesof T_h cells for antibody production and of IL-4-producing cellscompared with the response of control-treated mice. CD4⁺ T cellsof the anti-_µ-treated mice which respond to antigens ofP. chabaudi chabaudi maintained a T_h1 phenotype throughout primaryinfection, in contrast to control mice in which a sequentialappearance of T_h1 and T_h2 responses was observed. These datashow that T_h1 responses in anti-_µ-treated mice are sufficientto control parasitemia but not to eliminate an infection. Thedata further suggest that depletion of B cells by treatmentwith anti-_µ; antibodies reduces the generation of theT_h2 subset during a primary response to P. chabaudi chabaudi.