The potential for treg-enhancing therapies in nervous system pathologies.

While inflammation may not be the cause of disease, it is well known that it contributes to disease pathogenesis across a multitude of peripheral and central nervous system disorders. Chronic and overactive inflammation due to an effector T-cell-mediated aberrant immune response ultimately leads to tissue damage and neuronal cell death. To counteract peripheral and neuroinflammatory responses, research is being focused on regulatory T cell enhancement as a therapeutic target. Regulatory T cells are an immunosuppressive subpopulation of CD4+ T helper cells essential for maintaining immune homeostasis. The cells play pivotal roles in suppressing immune responses to maintain immune tolerance. In so doing, they control T cell proliferation and pro-inflammatory cytokine production curtailing autoimmunity and inflammation. For nervous system pathologies, Treg are known to affect the onset and tempo of neural injuries. To this end, we review recent findings supporting Treg's role in disease, as well as serving as a therapeutic agent in multiple sclerosis, myasthenia gravis, Guillain-Barre syndrome, Parkinson's and Alzheimer's diseases, and amyotrophic lateral sclerosis. An ever-broader role for Treg in the control of neurologic disease has been shown for traumatic brain injury, stroke, neurotrophic pain, epilepsy, and psychiatric disorders. To such ends, this review serves to examine the role played by Tregs in nervous system diseases with a focus on harnessing their functional therapeutic role(s).

Granulocyte-macrophage colony-stimulating factor neuroprotective activities in Alzheimer's disease mice.

We investigated the effects of granulocyte-macrophage colony stimulating factor (GM-CSF) on behavioral and pathological outcomes in Alzheimer's disease (AD) and non-transgenic mice. GM-CSF treatment in AD mice reduced brain amyloidosis, increased plasma Aß, and rescued cognitive impairment with increased hippocampal expression of calbindin and synaptophysin and increased levels of doublecortin-positive cells in the dentate gyrus. These data extend GM-CSF pleiotropic neuroprotection mechanisms in AD and include regulatory T cell-mediated immunomodulation of microglial function, Aß clearance, maintenance of synaptic integrity, and induction of neurogenesis. Together these data support further development of GM-CSF as a neuroprotective agent for AD.

Neuromagnetic evidence of abnormal movement-related beta desynchronization in Parkinson's disease.

Parkinson's disease (PD) is a neurodegenerative disorder associated with debilitating motor, posture, and gait abnormalities. Human studies recording local field potentials within the subthalamic nucleus and scalp-based electroencephalography have shown pathological beta synchronization throughout the cortical-basal ganglia motor network in PD. Suppression of such pathological beta synchronization has been associated with improved motor function, which may explain the effectiveness of deep-brain stimulation. We used magnetoencephalography (MEG) to investigate neural population-level beta responses, and other oscillatory activity, during a motor task in unmedicated patients with PD and a matched group of healthy adults. MEG is a noninvasive neurophysiological technique that permits the recording of oscillatory activity during movement planning, execution, and termination phases. Each of these phases was independently examined using beamforming to distinguish the brain areas and movement phases, where pathological oscillations exist during motor control. Patients with PD exhibited significantly diminished beta desynchronization compared with controls prior to and during movement, which paralleled reduced alpha desynchronization. This study is the first to systematically investigate neural oscillatory responses in PD during distinct stages of motor control (e.g. planning, execution, and termination) and indicates that these patients have significant difficulty suppressing cortical beta synchronization during movement planning, which may contribute to their diminished movement capacities.

BL-1023 improves behavior and neuronal survival in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-intoxicated mice.

The therapeutic potential of BL-1023, a chemical combination of L-3,4-dihydroxyphenylalanine (L-DOPA) and gamma-aminobutyric acid (GABA), was investigated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) intoxicated mice. Such animals exhibit nigrostriatal degeneration, characteristic of human Parkinson's disease. Drug was administered during and after the development of MPTP-induced nigrostriatal lesions followed by measures of motor function and behavior, surviving nigrostriatal dopaminergic neurons and termini, and striatal dopamine levels. When administered after lesion development, BL-1023 improved motor function of MPTP-mice as measured by rotarod, total floor and vertical plane movements, and stereotypic movements in open field activity tests compared to MPTP-mice without treatment. This also paralleled modest nigral dopaminergic neuronal protection. Such significant improvements in motor function, behaviors, and dopaminergic neuronal numbers were not seen when BL-1023 was administered during MPTP-induced lesion development. The data demonstrate select abilities of BL-1023 to increase dopaminergic neuronal survival and improve motor function in MPTP-mice.

Randomized controlled phase II trial of glatiramer acetate in ALS.

The authors conducted a randomized controlled trial to test the safety and immunology of glatiramer acetate in ALS. Twenty treated patients were randomly assigned to daily or biweekly injections. Ten control patients were selected from another trial and followed up concurrently. Injection reactions were the only common adverse event (p = 0.01). Treated patients showed enhanced lymphocyte proliferation (p = 0.02). The safety profile and immune effects support conducting larger trials of dose selection and efficacy.

Delivery of Flt3 ligand (Flt3L) using a poloxamer-based formulation increases biological activity in mice.

Fms-like tyrosine kinase (Flt3L) is a potent stimulator of hematopoietic progenitor cell (HPC) expansion and mobilization; however, this requires 7-10 days of administration. We investigated whether sustained delivery of Flt3L using a poloxamer-based matrix (PG) could accelerate and/or improve the hematopoietic activity of Flt3L in mice. A single injection of PG-Flt3L stimulated significantly more rapid and greater HPC mobilization to the spleen and peripheral blood than the daily injection of Flt3L formulated in saline. Pharmacokinetic analysis demonstrated that the formulation of Flt3L in PG prolonged its elimination (Tbeta) half-life (2.3-fold) and increased its bioavailability (>two fold) and the time to maximum serum concentration (T(max)) (2.7-fold). Further, coadministration of G-CSF and PG-Flt3L allowed lower doses of Flt3L to be active, with significantly greater hematopoietic and mobilization activity, compared to the same total dose of G-CSF, Flt3L or G-CSF and Flt3L formulated in saline. These data demonstrate that formulation of Flt3L in PG significantly accelerates and increases HPC expansion and mobilization. The observation of increased bioactivity by PG-Flt3L in rodents suggests the potential for improved clinical efficacy of Flt3L by reducing the time required for HPC mobilization.

Flt3 ligand and granulocyte-macrophage colony-stimulating factor preferentially expand and stimulate different dendritic and T-cell subsets.

OBJECTIVE: Mechanisms of T-cell stimulation by Flt3 ligand (Flt3L) and granulocyte-macrophage colony-stimulating factor (GM-CSF) remain unclear. Herein, we compared the effects of Flt3L and GM-CSF on the expansion of dendritic cells (DC) and T-cell subsets and cytokine expression. METHODS: Naïve and effector/memory T cells were analyzed by flow cytometry (FC). CD4(+) and CD8(+) T cells and CD11c(+)CD11b(dull/-)(DC1) and CD11c(+)CD11b(+) (DC2) subsets were isolated and the frequency of IFN-gamma-, IL-12- (type 1) and IL-4-, IL-10 (type 2)-producing cells and cytokine mRNA expression evaluated. RESULTS: Flt3L expanded both DC1 and DC2 subsets with a significantly higher percentage and number of DC1 than DC2, while GM-CSF preferentially expanded the DC2 subset. Isolated DC1 from Flt3L-injected mice had significantly higher levels of IL-12 (p40) than IL-10, while the converse occurred with DC2. The numbers of naïve and memory T cells were elevated in mice that received Flt3L or GM-CSF. However, the number of memory CD4(+) and CD8(+) T cells was significantly increased in Flt3L as compared to GM-CSF cohorts. While GM-CSF increased the frequency of both type 1 and type 2 cytokine-producing cells, Flt3L significantly augmented the frequency of type 1 T cells. CONCLUSIONS: In contrast to GM-CSF, Flt3L preferentially induces the expansion of type 1 T cells. The mechanism of Flt3L-induced T-cell stimulation is associated with the expansion of the IL-12 (p40)-producing DC1 and memory T cells.

Comparison of the hematopoietic activity of flt-3 ligand and granulocyte-macrophage colony-stimulating factor acting alone or in combination.

The hematopoietic sequelae of intramuscular administration of flt-3 ligand (FL) and granulocyte-macrophage colony-stimulating factor (GM-CSF) alone, or in combination, were compared in BALB/c mice. Changes in hematopoiesis were measured in the marrow, spleen and blood using an in vitro colony-forming unit (CFU) assay and flow cytometrically (expression of CD34 and stem cell antigen (Sca)-1). FL administration was associated with a significant increase in the absolute number of CFU and CD34+ cells in the marrow and CFU, CD34+, Sca-1+, and CD34+ Sca-1+ cells in the spleen and blood. These data demonstrate that FL expands and mobilizes a range of hematopoietic progenitors. By comparison, GM-CSF administration was associated with a significant increase in the number of CFU in the spleen and a significant reduction in marrow CD34+, Sca-1+, and CD34+Sca-1+ cells. These data suggest that GM-CSF-driven expansion of CFU may be at the expense of more primitive cells. The pattern of progenitor cell expansion associated with FL + GM-CSF administration was similar to that of FL alone with the following exceptions. The numbers of spleen and blood CFU were significantly greater and the number of marrow CD34+Sca-1+ cells were significantly less, than with FL alone. These data suggest that co-administration of these cytokines may combine the expansion of the more primitive cell populations (associated with FL) with the expansion of the more mature CFU population (associated with GM-CSF) to yield a greater overall CFU expansion and elevation of CFU in the blood. However, increasing the expansion and mobilization of the relatively mature, rather than the more primitive, hematopoietic progenitors, may be of limited value as a mobilization strategy, if the goal is the expansion and isolation of increased numbers of "high-quality," primitive cells for transplantation.

Flt3 ligand enhances the immunogenicity of a gag-based HIV-1 vaccine.

Liposomes and Flt3 ligand (Flt3L), a ligand for the fms-like tyrosine kinase receptor Flt3/ FLK2, can augment the immune response to an HIV peptide vaccine. The HGP-30 peptide used in these studies is a synthetic peptide that corresponds to a highly conserved region of HIV-1 p17 gag (amino acids 86-115). Mice were immunized with HGP-30 or HGP-30 conjugated to keyhole limpet hemocyanin (KLH) and delayed-type hypersensitivity (DTH) responses, antibody (IgG) amount and antigen-specific proliferative responses by spleen cells were used to monitor the immune response. Daily injections of Flt3L prior to HGP-30 administration enhanced significantly an antigen-specific lymphocyte proliferation response when compared with Flt3L, HGP-30 alone or HGP-30 containing liposomes. Intravenous administration of HGP-30 was superior to intramuscular (i.m.) immunization for the induction of DTH responses. The HGP-30/KLH containing liposomes enhanced both DTH and antibody responses, while liposomes containing HGP-30 peptide elicited only T cell responses. In these studies, either Flt3L or liposomes increased DTH responses compared with the i.m. injection of the HGP-30 vaccine alone.

Altered development of intestinal intraepithelial lymphocytes in P-glycoprotein-deficient mice.

Intraepithelial lymphocytes (IEL) that reside in the intestinal epithelium are known to exhibit phenotypic and functional characteristics that are distinct from other T cells. We have recently shown that peripheral T cells exclusively express an isoform of P-glycoprotein (P-gp) encoded by the mdr1a gene, but do not require mdr1a expression for normal proliferative, cytokine, or cytotoxic responses. In the present study, we have used mdr1-type knockout (KO) mice to demonstrate that IEL also utilize mdr1a, but only preferentially, in that the mdr1b isoform can be expressed in the absence of mdr1a expression. We also report that a high level of P-gp activity appears to be necessary for the normal development of certain IEL subpopulations. In specific, while the total number of IEL was relatively unaffected by the absence of mdr1a expression, the proportions of CD8 alpha beta and TCR alpha beta+ IEL increased significantly in mdr1a and mdr1a/b KO mice at the expense of CD8 alpha alpha and TCR gamma delta+ IEL, respectively. Moreover, these subset alterations also appeared to have functional consequences, in that proliferative, IL-2, and IFN-gamma responses of IEL from KO mice were distinct from those of normal IEL. In summary, our data suggest that mdr1a expression is required for the development of certain IEL subpopulations, most notably TCR gamma delta+ cells, and thereby indirectly influences the balance of T cell subsets in the intestinal epithelium.

Idiosyncratic alterations of TCR size distributions affecting both CD4 and CD8 T cell subsets in aging mice.

We have used a spectratyping method, which displays the size distribution for the complementarity-determining region 3 (CDR3) for T cells utilizing a specific TCR-Vbeta gene, to examine the effects of aging on the TCR repertoire of (BALB/c x C57BL/6)F1 hybrid mice. Although the size distributions from T cells of 8-month-old mice were typically symmetrically shaped around one or two bands of intermediate size, spectratypes from mice 16 or 24 months of age were frequently distorted, with specific size classes either over- or underrepresented compared to normal young controls. Each of 12 mice tested at 16 or 24 months of age had a skewed spectratype for at least one of the 24 Vbeta families examined, and some mice had more than 50% of their spectratypes skewed significantly, as judged by a chi2 test. Comparable age-associated skewing of the T cell repertoire occurred in the CD4 and CD8 subsets, and every mouse over 16 months of age exhibited at least one skewed Vbeta family in both the CD4 and CD8 populations. Although the mice were genetically identical and raised in common facilities, their spectratype patterns were nonetheless idiosyncratic: i.e., the specific set of abnormalities was distinct for each individual old mouse. Whether these distortions of the TCR repertoire in middle-aged and older mice lead to alterations in immune function remains to be determined.

Stimulation via the CD43 coreceptor augments T cell proliferation during the early phase of antigen-induced activation.

R2/60 is a monoclonal antibody (mAb) that recognizes the murine CD43 molecule expressed on mature T cells, developing thymocytes, and a subset of bone marrow hematopoietic stem cells. Recent studies using R2/60 demonstrate that CD43 is a costimulatory receptor involved in the activation of murine T cells. In the present study we have examined the kinetics of CD43-mediated costimulation in murine T cell populations including purified CD4+ and CD8+ peripheral T cells, and T cells activated in antigen-induced mixed-lymphocyte cultures. In each population, CD43 stimulation significantly enhanced T cell proliferation compared to non-CD43 activated cultures. CD43 costimulation was greatest in conjunction with suboptimal CD3 stimulation, and was most efficient during the early phase of antigen-driven activation, suggesting that under normal biologic conditions the role of CD43 may be to augment T cell responses early in immune activation.

Aging, immunity and neuroendocrine hormones.

Recent evidence indicates that the neuroendocrine and immune systems are intimately integrated into one system that provides a complex homeostatic network. Disruption of one system by extrinsic factors such as stress or antigenic exposure usually has consequences on the other. With advancing age, a progressive disruption can be observed in both systems which may have profound implications with respect to age-associated pathologies, including autoimmunity. In this review evidence is summarized which supports the hypothesis that neuroendocrine factors influence the age-associated decline of the immune system.

CD43 is a murine T cell costimulatory receptor that functions independently of CD28.

Costimulation mediated by the CD28 receptor has been shown to play an important role in the development of a vigorous T cell immune response. Nevertheless, CD28-deficient mice can mount effective T cell-dependent immune responses. These data suggest that other costimulatory molecules may play a role in T cell activation. In a search for other costimulatory receptors on T cells, we have characterized a monoclonal antibody (mAb) that can costimulate T cells in the absence of accessory cells. Similar to CD28 antibodies, this mAb, R2/60, was found to synergize with T cell receptor engagement in inducing proliferation. Independent ligation of CD3 and the ligand recognized by R2/60 results in T cell proliferation, suggesting that the two molecules do not have to colocalize to activate the R2/60 costimulatory pathway. R2/60 does not react with CD28, and furthermore, R2/60 costimulates in a CD28-independent fashion since the mAb costimulates T cells from the CD28-deficient mice as well as wild-type mice. Expression cloning of the R2/60 antigen identified the ligand as murine CD43. Together, these data demonstrate that CD43 can serve as a receptor on T cells that can provide CD28-independent costimulation.

A novel marker of murine bone marrow hematopoietic stem cells that is expressed on peripheral T cells and is associated with a functionally important molecule on activated cytotoxic T lymphocytes.

A MAb (R2/60) has been isolated that defines a novel lymphocyte marker of murine T cells. The determinant recognized by MAb R2/60 is present on a subset of bone marrow (BM) hematopoietic stem cells, on adult thymocytes, on peripheral T cells (both resting and activated), and on murine T cell tumor lines, although it is not expressed on mature B cells. In immunoprecipitation studies using radiolabeled membrane lysates from adult thymocytes, MAb R2/60 precipitated a 44-kDa membrane-bound dimer. Functionally, MAb R2/60 mediated antigen-independent cell lysis by activated CTLs, and by CTL clones, when bridged to Fc receptor-bearing target cells; however, binding of MAb R2/60 to effector cells prior to cytotoxic assays did not inhibit target cell lysis by CTLs, suggesting that the R2/60 determinant is involved in transmembrane signaling to already activated CTLs, but that it is not involved in target cell adhesion or antigen recognition. Moreover, direct stimulation of T cells by MAb R2/60 in the absence of additional stimuli did not induce cell proliferation, further implying that the R2/60 determinant is functionally involved in the effector rather than the inductive phase of the T cell response.

Functional heterogeneity of murine intestinal intraepithelial lymphocytes: studies using TCR-alpha beta+ IEL lines and fresh IEL isolates reveal multiple cytotoxic subsets differentiated by CD5, CD8 alpha/alpha, and CD8 alpha/beta expression.

Three T-cell lines, isolated from murine small intestine epithelia, have been studied with respect to phenotypic properties and cytotoxic activity. All lines were TCR-alpha beta+, Thy-1+, CD3+, CD4-, CD8+ but differed in that one line was CD8 alpha/alpha+, CD5-; one line was CD8 alpha/alpha+, CD5+; and one line was CD8 alpha/beta+, CD5+. Both the CD8 alpha/alpha+, CD5-, and the CD8 alpha/beta+, CD5+ lines lysed antigen-bearing target cells; however, the latter line also spontaneously lysed natural killer (NK)-sensitive target cells. The CD8 alpha/alpha+, CD5+ IEL line was nonlytic for antigen-bearing target cells, for NK-sensitive target cells, and in assays that detect lytic activity regardless of specificity. Three-color flow cytometric analyses of intestinal intraepithelial lymphocytes (IEL) in freshly-extracted preparations indicated that cells with the phenotypes of the three IEL lines are normally present in the murine intestine epithelium, and revealed considerable variability in the distribution and density of CD5 expression on murine IEL. In freshly extracted IEL depleted of CD5 or CD8 beta by cell sorting, cytotoxicity was found to reside both within the CD5+ and the CD5- IEL subsets, as well as in the CD8 beta-depleted (i.e., CD8 alpha/alpha+) subset. These findings demonstrate: that cytotoxicity of murine IEL resides among multiple phenotypic subsets; that the distribution and density of CD5 on IEL is more complex than previously described; and that T-cell lines of IEL origin are valuable for dissecting functional properties of specific IEL subsets, particularly those that constitute a small proportion of the total IEL.

Phenotype and TCR gamma delta variable gene repertoire of intestinal intraepithelial lymphocytes in wild mice (Mus musculus domesticus): abundance of V gamma 1 transcripts and extensive delta gene diversity.

In order to study murine intestinal intraepithelial lymphocytes (IEL) independent of factors imparted by conditions of laboratory housing and breeding, and to provide a basis for comparison of IEL studies between inbred and outbred mouse populations, IEL from the domestic house mouse, Mus musculus domesticus, were analyzed by flow cytometric analyses using mAbs to murine lymphocyte markers, and by polymerase chain reaction to study the TCR gamma and delta V gene repertoires. The majority of IEL in wild mice were CD3+, CD8+CD4- T cells. CD4+CD8- also were present in IEL isolates from wild mice, although at low numbers. Among IEL, but not T cells from the spleen or lymph nodes, there was a notable lack of Thy-1 expression, a preponderance of CD8 alpha alpha + T cells, and a relatively high ratio (3:1) of TCR gamma delta + T cells over TCR alpha beta + T cells, suggesting that some IEL in wild mice may develop via an extrathymic pathway similar to that described for laboratory mice. Analyses of the IEL gamma and delta variable genes revealed rearrangements of three of six V region gamma genes (V gamma 1, V gamma 2, and V gamma 5), with an abundance of V gamma 1 transcripts as determined by Northern blot analyses. For the delta gene, rearrangement of five of seven V region elements had occurred (V delta 2, V delta 3, V delta 4, V delta 5, and V delta 6).(ABSTRACT TRUNCATED AT 250 WORDS)

Immunopathologic characterization of typhlitis and colitis in juvenile HLA-B27 transgenic rats.

Adult HLA-B27 transgenic rats carrying high copy numbers of human HLA-B27 and beta 2-microglobulin genes spontaneously develop spondyloarthropathy and enterocolitis comparable to human HLA-B27-associated disease. In this investigation, juvenile HLA-27 transgenic rats were utilized to study incipient immunopathologic events in HLA-B27-associated gastrointestinal inflammation. Flow cytometric analysis of peripheral lymphocytes demonstrated distinctive differences in HLA-B27 protein expression and prompted the division of these transgenic rodents into 2 groups: HLA-B27hi and HLA-B27lo. The HLA-B27hi group, which represented 60% of the rats (aged 8-12 weeks) had inflammation in the colon, anorectal junction and cecum but spared the small intestine. Inflammation coexisted with high levels of surface HLA-B27 expression by hematopoietically derived cells as determined by immunofluorescence staining and flow cytometric analysis of intestinal lymphocytes. Inflammation, which was most intense in the cecum and anorectal junction, was characterized by mixed cellular infiltrate, crypt hyperplasia, transepithelial migration of neutrophils and a reduction in goblet cells. T lymphocytes, particularly CD4+ T cells, predominated over other lymphocytes in the inflammatory infiltrate of the lamina propria. Conversely, no inflammation was evident at any level of the gastrointestinal tract in the HLA-B27lo group (8 weeks of age) which constituted 40% of the juvenile transgenic rats. These animals all expressed low level of HLA-B27 protein. Collectively, these data indicate that HLA-B27 protein expression increases dramatically from 8 to 12 weeks of age and that the level of protein expression and intestinal inflammation are interrelated. These associated gastrointestinal events occur during puberty and thus we speculate that the high level of protein expression may be hormonally mediated.

T cell receptor delta gene repertoire and diversity of intestinal intraepithelial lymphocytes in athymic mice.

T cell receptor (TCR) delta gene rearrangements in intestinal intraepithelial lymphocytes (IEL) were studied in athymic radiation chimeras using polymerase chain reaction (PCR) and sequence analysis of DNAs spanning the variable (V), diversity (D), and junctional (J) genes. In both thymus-bearing and athymic mice, IEL delta gene rearrangements occurred for V delta 3, V delta 4, V delta 5 and V delta 6. V-D-J junctional-site sequence analyses of cloned DNAs from rearranged IEL delta genes in athymic mice revealed a predominance of in-frame rearrangements; junctional diversity consisting of nucleotide removal from V, D and/or J genes; N segment nucleotide insertions; and high overall gene diversity. Evaluation of PCR-amplified cDNAs made from IEL RNA indicated that all four rearranged V delta genes were expressed in IEL from athymic mice. The high diversity observed at the gene level also was present in amino acid sequences encoded by the V-D-J region of IEL delta genes in athymic mice. These data demonstrate that there is extensive diversity of rearranged delta genes in IEL which develop extrathymically, and suggest that the delta chain of IEL TCR-gamma delta+ T cells has the potential for interactions with polymorphic structures.

Peripheral engraftment of fetal intestine into athymic mice sponsors T cell development: direct evidence for thymopoietic function of murine small intestine.

Adult athymic, lethally irradiated, F1-->parent bone marrow-reconstituted (AT x BM) mice were engrafted bilaterally with day 16-18 fetal intestine or fetal thymus into the kidney capsule and were studied for evidence of peripheral T cell repopulation of 1-12 wk postengraftment. Throughout that time period, both types of grafts were macroscopically and histologically characteristic of differentiated thymus or intestine tissues, respectively. Beginning at week 2 postengraftment, clusters of lymphocytes were present within intestine grafts, particularly in subepithelial regions and in areas below villus crypts. As determined by immunofluorescence staining and flow cytometric analyses, lymphocytes from spleen and lymph nodes of sham-engrafted mice (AT x BM-SHAM) were essentially void of T cells, whereas in AT x BM thymus-engrafted (AT x BM-THG) mice, which served as a positive control for T cell repopulation, normal levels of T cells were present in spleen and lymph nodes by week 3 postengraftment, and at times thereafter. Most striking, however, was the finding that T cell repopulation of the spleen and lymph nodes occurred in AT x BM fetal intestine-engrafted (AT x BM-FIG) mice beginning 3 wk postengraftment. Based on H-2 expression, peripheral T cells in AT x BM-FIG mice were of donor bone marrow origin, and consisted of CD3+, T cell receptor (TCR)-alpha/beta+ T cells with both CD4+8- and CD4-8+ subsets. Peripheral T cells in AT x BM-FIG mice were functionally mature, as demonstrated by their capacity to proliferate after stimulation of CD3 epsilon. Moreover, alloreactive cytotoxic T lymphocytes were generated in primary in vitro cultures of spleen cells from AT x BM-FIG and AT x BM-THG mice, though not in spleen cell cultures from AT x BM-SHAM mice. Histologic studies of engrafted tissues 3-4 wk postengraftment demonstrated that thymus leukemia (Tl) antigens were expressed on epithelial surfaces of intestine grafts, and that both TCR-alpha/beta+ and TCR-gamma/delta+ lymphocytes were present in intestine grafts. Collectively, these findings indicate that the murine small intestine has the capacity to initiate and regulate T cell development from bone marrow precursors, thus providing a mechanism by which extrathymic development of intestine lymphocytes occur.