Antigen-specific regulatory T-cell responses to intestinal microbiota.

The mammalian gastrointestinal tract can harbor both beneficial commensal bacteria important for host health, but also pathogenic bacteria capable of intestinal damage. It is therefore important that the host immune system mount the appropriate immune response to these divergent groups of bacteria-promoting tolerance in response to commensal bacteria and sterilizing immunity in response to pathogenic bacteria. Failure to induce tolerance to commensal bacteria may underlie immune-mediated diseases such as human inflammatory bowel disease. At homeostasis, regulatory T (Treg) cells are a key component of the tolerogenic response by adaptive immunity. This review examines the mechanisms by which intestinal bacteria influence colonic T-cells and B-cell immunoglobulin A (IgA) induction, with an emphasis on Treg cells and the role of antigen-specificity in these processes. In addition to discussing key primary literature, this review highlights current controversies and important future directions.

Intra-articular injection of the selective cyclooxygenase-2 inhibitor meloxicam (Mobic) reduces experimental osteoarthritis and nociception in rats.

OBJECTIVE: To study the effect of intra-articular injection of meloxicam (Mobic) on the development of osteoarthritis (OA) in rats and examine concomitant changes in nociceptive behavior and the expression of mitogen-activated protein kinases (MAPKs) in articular cartilage chondrocytes. METHODS: OA was induced in Wistar rats by right anterior cruciate ligament transection (ACLT); the left knee was not treated. The OA + meloxicam (1.0 mg) group was injected intra-articularly in the ACLT knee with 1.0 mg of meloxicam once a week for 5 consecutive weeks starting 5 weeks after ACLT. The OA + meloxicam (0.25 mg) group was treated similarly with 0.25 mg meloxicam. The sham group underwent arthrotomy only and received vehicle of 0.1 mL sterile 0.9% saline injections, whereas the naive rats in meloxicam-only groups were treated similarly with 1.0- and 0.25-mg meloxicam. Nociception was measured as secondary mechanical allodynia and hind paw weight-bearing distribution at before (pre-) and 5, 10, 15, and 20 weeks post-ACLT. Histopathology of the cartilage and synovia was examined 20 weeks after ACLT. Immunohistochemical analysis was performed to examine the effect of meloxicam on MAPKs (p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK)) expression in the articular cartilage chondrocytes. RESULTS: OA rats receiving intra-articular meloxicam treatment showed significantly less cartilage degeneration and synovitis than saline-treated controls. Nociception were improved in the OA + meloxicam groups compared with the OA group. Moreover, meloxicam attenuated p38 and JNK but enhanced ERK expression in OA-affected cartilage. CONCLUSIONS: Intra-articular injection of meloxicam (1) attenuates the development of OA, (2) concomitantly reduces nociception, and (3) modulates chondrocyte metabolism, possibly through inhibition of cellular p38 and JNK, but enhances ERK expression.

Integration of CNS survival and differentiation by HIF2α.

Hypoxia-inducible factor (HIF) 1α and HIF2α and the inhibitor of apoptosis survivin represent prominent markers of many human cancers. They are also widely expressed in various embryonic tissues, including the central nervous system; however, little is known about their functions in embryos. Here, we show that zebrafish HIF2α protects neural progenitor cells and neural differentiation processes by upregulating the survivin orthologues birc5a and birc5b during embryogenesis. Morpholino-mediated knockdown of hif2α reduced the transcription of birc5a and birc5b, induced p53-independent apoptosis and abrogated neural cell differentiation. Depletion of birc5a and birc5b recaptured the neural development defects that were observed in the hif2α morphants. The phenotypes induced by HIF2α depletion were largely rescued by ectopic birc5a and birc5b mRNAs, indicating that Birc5a and Birc5b act downstream of HIF2α. Chromatin immunoprecipitation assay revealed that HIF2α binds to birc5a and birc5b promoters directly to modulate their transcriptions. Knockdown of hif2α, birc5a or birc5b reduced the expression of the cdk inhibitors p27/cdkn1b and p57/cdkn1c and increased ccnd1/cyclin D1 transcription in the surviving neural progenitor cells. The reduction in elavl3/HuC expression and enhanced pcna, nestin, ascl1b and sox3 expression indicate that the surviving neural progenitor cells in hif2α morphants maintain a high proliferation rate without terminally differentiating. We propose that a subset of developmental defects attributed to HIF2α depletion is due in part to the loss of survivin activity.

Electro-optic guided-to-radiation mode conversion in annealed proton-exchanged PPLN waveguides.

We report the design and experimental demonstration of electro-optically active TM-guided to TE-radiation mode converters in annealed proton-exchanged (APE) periodically poled lithium niobate (PPLN) channel waveguides in telecom S-C-L bands (1495-1640 nm). A maximum mode conversion efficiency of >95%/cm was obtained at 1520 nm from a 24-µm-period APE PPLN waveguide under an electro-optic (EO) field of ~6.3 V/µm at 35°C. This efficiency has been enhanced by a factor of >4.6 over a waveguide built in the single-domain (unpoled) LiNbO3; it is also to the best of our knowledge the most efficient guided-to-radiation (GTR) mode converter ever reported based on LiNbO3 on-axis waveguides. A conversion bandwidth of ~250 nm was also observed from this EO GTR mode converter.

Glucosamine sulfate reduces experimental osteoarthritis and nociception in rats: association with changes of mitogen-activated protein kinase in chondrocytes.

OBJECTIVE: To study the effects of oral glucosamine sulfate on the development of osteoarthritis (OA) and to examine concomitant changes in the nociceptive behavior of rats. METHODS: OA was induced in Wistar rats by anterior cruciate ligament transection (ACLT) of the right knee; the left knee was untreated. The OA+glucosamine group received oral glucosamine sulfate (250 mg/kg/day) in a 2-g wafer once a day for 10 consecutive weeks starting at week 5 after ACLT. The OA group was treated as above with 2-g wafers (placebo). The control group of naïve rats received 2-g wafers only. The glucosamine alone group comprised naïve rats receiving glucosamine sulfate only. Nociceptive behavior (mechanical allodynia and weight-bearing distribution of hind paws) during OA development was analyzed pre- and 3, 6, 9, 12, 15, and 18 weeks post-ACLT. Macroscopic and histologic studies were then performed on the cartilage and synovia. Immunohistochemical analysis was performed to examine the effect of glucosamine on expression of mitogen-activated protein kinases (MAPKs) in the articular cartilage chondrocytes. RESULTS: OA rats receiving glucosamine showed a significantly lower degree of cartilage degeneration than the rats receiving placebo. Glucosamine treatment also suppressed synovitis. Mechanical allodynia and weight-bearing distribution studies showed significant improvement in the OA+glucosamine group as compared to the OA group. Moreover, glucosamine attenuated p38 and c-Jun N-terminal kinase (JNK) but increased extracellular signal-regulated kinase 1/2 (ERK) expression in OA-affected cartilage. CONCLUSION: Our results indicate that treatment with oral glucosamine sulfate in a rat OA model (1) attenuates the development of OA, (2) concomitantly reduces nociception, and (3) modulates chondrocyte metabolism, possibly through inhibition of cell p38 and JNK and increase of ERK expression.

Intra-articular magnesium sulfate (MgSO4) reduces experimental osteoarthritis and nociception: association with attenuation of N-methyl-D-aspartate (NMDA) receptor subunit 1 phosphorylation and apoptosis in rat chondrocytes.

OBJECTIVE: To study the effects of intra-articular injection of magnesium sulfate (MgSO(4)) on the development of osteoarthritis (OA) and to examine concomitant changes in the nociceptive behavior of rats. METHODS: OA was induced in Wistar rats with intra-articular injection of collagenase (500 U) in the right knee; the left knee was left untreated. In the OA+MgSO(4) group (n=7), the treated knee was injected with 500-microg (0.1-ml) MgSO(4) twice a week for 5 consecutive weeks starting at 1 week after collagenase injection; in the OA group (n=7), the same knee was injected with the same amount of physiological normal saline. In the MgSO(4) group (n=6), naïve rats received only MgSO(4) injections; in the control group (n=6), naïve rats received only physiological normal saline injections. Nociceptive behavior (mechanical allodynia and thermal hyperalgesia) on OA development was measured before and at 1, 2, 4, 6, and 8 weeks after collagenase injection, following which the animals were sacrificed. Gross morphology and histopathology were examined in the femoral condyles, tibial plateau, and synovia. Immunohistochemical analysis was performed to examine the effect of MgSO(4) on N-methyl-D-aspartate (NMDA) receptor subunit 1 phosphorylation (p-NR1) and apoptosis in the articular cartilage chondrocytes. RESULTS: OA rats receiving intra-articular MgSO(4) injections showed a significantly lower degree of cartilage degeneration than the rats receiving saline injections. MgSO(4) treatment also suppressed synovitis. Mechanical allodynia and thermal hyperalgesia showed significant improvement in the OA+MgSO(4) group as compared to the OA group. Moreover, MgSO(4) attenuated p-NR1 and chondrocyte apoptosis in OA-affected cartilage. CONCLUSIONS: Our results indicate that local intra-articular administration of MgSO(4) following collagenase injection in an experimental rat OA model (1) modulates chondrocyte metabolism through inhibition of cell NMDA receptor phosphorylation and apoptosis, (2) attenuates the development of OA, and (3) concomitantly reduces nociception.

The role of TCR specificity in naturally arising CD25+ CD4+ regulatory T cell biology.

CD25+ CD4+ T cells (TR) are a naturally arising subset of regulatory T cells important for the preservation of self-tolerance and the prevention of autoimmunity. Although there is substantial data that TCR specificity is important for TR development and function, relatively little is known about the antigen specificity of naturally arising TR. Here, we will review the available evidence regarding naturally arising TR TCR specificity in the context of TR development, function, and homeostasis.

Bacteria ascend to liver from the bilioenteric conduit after choledochojejunostomy in the cholestatic rat.

The high incidence of postoperative cholangitis in children with clinical restoration of bile flow after Roux-Y choledochojejunostomy (RYCJ) assumed the concept of a direct ascending cholangitis caused by pathogens in the intestine, into the intrahepatic bile duct via the porta hepatis. It is also well known that jaundiced animals (patients) are more susceptible to infections of the bile ducts following the procedure of bilioenteric anastomosis. An animal experiment was conducted to compare quantitative bacterial cultures of the choledochojejunostomy area and the liver 24 hours after Escherichia coli (ATCC 25922) or sterile normal saline was injected into the bilioenteric conduit (BEC), following RYCJ in rats with or without the proceeding bile duct ligation. A significant increase of E. coli of the same strain (ATCC 25922), that we injected into the BEC, was proved with pulse-field gel electrophoresis (PFGE) and shown in the liver of the jaundiced rats receiving E. coli (ATCC 25922), compared to that in the nonjaundiced rats with normal saline treatment. It is concluded that bacteria often ascend early to the liver from the BEC following RYCJ. This ascending cholangitis model might be produced for further studies.

Ascending cholangitis provokes IL-8 and MCP-1 expression and promotes inflammatory cell infiltration in the cholestatic rat liver.

BACKGROUND/PURPOSE: Postoperative cholangitis is one of the most common complications after bile duct reconstruction. The pathogenesis and early consequences of ascending cholangitis still are unidentified. METHODS: Male Sprague-Dawley rats were divided into 5 treatment groups: control (n = 4), blood sampling and liver biopsy only; group I, [BDL/Eschericha coli; n = 6], ligation of common bile duct (BDL) for a week, followed by Roux-en-Y choledochojejunostomy (RYCJ) and injection of E coli (ATCC 25922) into Roux limb after 24 hours; group II, [BDL/NS; n = 5], same procedures as in group I, with injection of normal saline (NS) into Roux limb; group III, [SBDL/E coli; n = 6], primary RYCJ was constructed 1 week after sham ligation of common bile duct (SBDL) followed by the same treatment as group I; Group IV, [SBDL/N.S; n = 6], same procedures as in group III, but injecting NS into Roux limb. All animals were killed after 24 hours of treatment. Blood was sampled for culture and serum cytokine levels. The liver was harvested for quantitative bacterial culture, as well as for MCP-1, interleukin (IL)-8 (CINC in the rat) and transforming growth factor beta1 mRNA expression by reverse transcriptase polymerase chain reaction (RT-PCR) and for immunohistochemistry. The choledochojejunostomy was resected for culture. Serum cytokine levels were detected by ELISA kits. RESULTS: A significant increase of E coli ATCC 25922, occurred in the livers of group I rats, compared with group IV (P =.037). MCP-1 expression increased in all groups, compared with control (P =.000). The IL-8 mRNA expression was significantly higher in group I than in control (P =.021). The expression of TGF-beta1 mRNA was similar among the groups (P =.361), consistent with the immunohistochemistry results. The serum MCP-1 and IL-8 levels were higher in the 4 groups than in the control (P =.000) and were significantly higher in group I than in group IV (P =.001). CONCLUSIONS: This study found that a significant colonization of E coli of the same strain was present in the cholestatic rat liver injected into the Roux limb, which was associated with a higher expression of liver MCP-1 and IL-8 mRNA, a significant increase of serum MCP-1 and IL-8, and a more evident inflammatory cell infiltration into the porta hepatis.

Changes in bacterial concentration in the liver correlate with that in the hepaticojejunostomy after bile duct reconstruction: implication in the pathogenesis of postoperative cholangitis.

Postoperative cholangitis is a frequent and unpredictable complication of unknown etiology following bile duct reconstruction (BDR), particularly for biliary atresia. This study was undertaken to correlate the growth of bacteria in the hepaticojejunostomy with that in the liver after BDR. Quantitative bacterial culture was done on the specimens taken from the liver and from the hepaticojejunostomy at 1 week (group 1, n = 7), 1 month (group 2, n = 7), and 2 months (group 3, n = 7) following BDR with Roux-en-Y hepaticojejunostomy in piglets after 2 weeks of common bile duct ligation. The histological examination of the liver and the hepaticojejunostomy, as well as serial monitoring of hemogram and liver function tests, were performed to correlate the findings with the bacterial concentration of the liver and the hepaticojejunostomy following BDR. The bacterial concentration of the hepaticojejunostomy, expressed as log10 colony-forming units per gram (log10 CFU/g) of the hepaticojejunostomy, showed a progressive decrease from 8.38 +/- 1.36 in group 1, 7.07 +/- 2.54 in group 2, to 3.56 +/- 1.31 in group 3 (p = 0.001). The log10 CFU/g of the liver also showed a progressive decrease from 5.02 +/- 1.59 in group 1, 3.16 +/- 1.56 in group 2, to 2.19 +/- 1.09 in group 3 (p = 0.006). There was a significant positive correlation of the log10 CFU/g of the liver (n = 21) with that of the hepaticojejunostomy (n = 21) following BDR (r = 0.600, p = 0.004). Most of the infectious pathogens isolated from the liver were also isolated from the hepaticojejunostomy. The changes in hemoglobin, bilirubin, albumin, and ammonia significantly correlated with the changes of the bacterial concentration of the liver. The results of the study suggests that hepatic bacterial proliferation after BDR is significantly affected by microbial overgrowth in the bilioenteric anastomosis and is associated with deteriorated liver function and hemogram.

Infantile hypertrophic pyloric stenosis presenting as pseudo-Bartter's syndrome and seizures: report of one case.

We report a hypertrophic pyloric stenosis case with an unusual initial presentation of seizures and Bartter's syndrome like symptoms. This case suffered from vomiting, diarrhea and poor appetite for several days, and seizures developed after these symptoms. From laboratory tests, hypochloremic and hypokalemic metabolic alkalosis associated with hyperreninemia, hyperaldosteronism and normal blood pressure were noted. Pseudo-Bartter's syndrome was diagnosed through these clinical and laboratory tests. Although the first abdominal echo was negative, we still speculated about the peculiar symptoms of vomiting and it's relationship to pseudo-Bartter's syndrome. After all, we found the hypertrophic pyloric stenosis through an upper gastrointestinal series. From these experiences, we postulated that it's very important to put the hypertrophic pyloric stenosis into the differential diagnosis of pseudo-Batter's syndrome.

Necrotizing enterocolitis in newborn: nine years' experience.

BACKGROUND: Necrotizing enterocolitis (NEC) is the most significant acquired gastrointestinal (GI) emergency in the neonatal intensive care unit. METHODS: We sought to gain a clinical perspective on NEC by reviewing the records of NEC patients over a 9-year period. The case histories of 22 infants with NEC treated from September 1, 1986 to September 1, 1995 were reviewed. RESULTS: Mean gestational age was 32 weeks and mean birth weight was 1774 grams. Eighteen percent were full term babies and 82% were premature. Average age at the onset of NEC was 11 days. The most common clinical manifestations were abdominal distension (100%), gastric retention (64%), unstable vital signs (59%) and Guaiac-positive vomitus or stool (36%). Sixteen cases (73%) were classified as stage III NEC, which has the highest mortality and/or morbidity. CONCLUSION: Early identification and management are critical to improve the outcome of NEC.

Endoscopic sympathectomy treatment for craniofacial hyperhidrosis.

OBJECTIVE: To present endoscopic T-2 sympathectomy as a minimally invasive therapy for craniofacial hyperhidrosis (CH). DESIGN: Follow-up study of 30 patients with CH treated by the new method in a 4-year period. The duration of follow-up was from 8 to 44 months (mean, 15 months). SETTING: University hospital. PATIENTS: Thirty consecutive patients with CH (18 men, 12 women) treated by the new method. All patients were essentially in good health except that they suffered from distressing CH to the extent that their daily activities were often disturbed. Their ages ranged from 7 to 63 years (mean age, 42.8 years). INTERVENTION: Endoscopic sympathectomy on both sides was carried out in a 1-stage operation for all patients. MAIN OUTCOME MEASURES: The patients were interviewed 1 week and then 3 months after surgery and then followed up by telephone interview about the alleviation or recurrence of CH and complications. RESULTS: All of the treated patients obtained a satisfactory alleviation of CH. One case was complicated by a mild and transient ptosis of the left eye. No recurrence of CH was noticed during the follow-up period. CONCLUSIONS: This therapeutic procedure is minimally invasive and effective. It causes minimal discomfort and was associated with no major complications in this series. The patients require only an overnight hospital stay and the operation scars are small. Endoscopic sympathectomy has proven to be an effective method in treating patients with distressing CH.

Minimally invasive surgery: video endoscopic thoracic sympathectomy for palmar hyperhidrosis.

Palmar hyperhidrosis (PH) is a common disorder in Taiwan. It often causes social embarrassment and occupational handicaps. So far, there has been no satisfactory treatment for PH. In 1990, we first developed a minimally invasive technique: video endoscopic sympathectomy to treat PH. The procedure has subsequently proven to be a standard treatment for PH. In this study, a survey of 9988 cases of PH patients from 17 hospitals in Taiwan treated by this method during the past 5 years is presented. Although there were some variations in the model of anaesthesia, technique and extent of sympathectomy, the postoperative results were generally satisfactory. Both sides of sympathectomy were mostly accomplished within half an hour in one stage. The operative scars were tiny and concealed in the axillary region. The patients were discharged from the hospital after an overnight stay. Complications such as pneumothorax, haemothorax (0.3%) or Horner's syndrome (0.1%) were rare. There was no surgical mortality in this series. The most common complication was compensatory hyperhidrosis which was usually mild to moderate and tolerable after reassurance. The recurrence rate of PH was approximately 1% in the first year and less than 3% during the 3 years of follow up. Intraoperative monitoring of palmar skin temperature (PST) was advocated to confirm an adequate sympathectomy warranting a definite result. En bloc ablation of T2 segment invariably resulted in a rise of PST to about 2 degrees C and was considered as an adequate extent of sympathectomy for PH. The refined technique was extended to treat young children with PH and patients with craniofacial hyperhidrosis. The therapeutic results were generally excellent with minimal morbidity and rare recurrence. It is concluded that video endoscopic en bloc T2 sympathectomy is a simple, minimally invasive and effective treatment for both adults and children with PH and also for patients with craniofacial hyperhidrosis.

Juvenile fibromatosis of the posterior mediastinum with intraspinal extension.

Chest radiography, CT, and MR imaging were performed in a 3-year-old girl who had posterior mediastinal fibromatosis with transforaminal intraspinal and chest wall extension. Chest radiographs and CT scans showed a slow-growing, noncalcified but locally aggressive left paravertebral mass. The mass was slightly hyperintense relative to muscle on both T1-weighted and fast spin-echo T2-weighted MR images.

Genetic susceptibility to Leishmania: IL-12 responsiveness in TH1 cell development.

The genetic background of T lymphocytes influences development of the T helper (TH) phenotype, resulting in either resistance or susceptibility of certain mouse strains to pathogens such as Leishmania major. With an in vitro model system, a difference in maintenance of responsiveness of T cells to interleukin-12 (IL-12) was detected between BALB/c and B10.D2 mice. Although naive T cells from both strains initially responded to IL-12, BALB/c T cells lost IL-12 responsiveness after stimulation with antigen in vitro, even when cocultured with B10.D2 T cells. Thus, susceptibility of BALB/c mice to infection with L. major may derive from the loss of the ability to generate IL-12-induced TH1 responses rather than from an IL-4-induced TH2 response.

Epidermolysis bullosa letalis with pyloric atresia in an infant.

Epidermolysis bullosa (EB) is a group of inherited diseases, that are characterized by vesiculobullous lesions that arise in response to minimal trauma or friction. The three major groups of EB differ according to the ultrastructural level of cleavage namely: simplex (epidermolytic), junctional and dystrophic (dermolytic). The combination of EB and pyloric atresia in rare and there is a definite association between them. We report a baby boy who died epidermolysis bullosa complications despite successful surgical correction of this pyloric atresia.

Tissue-specific targeting of cytokine unresponsiveness in transgenic mice.

The ubiquitous cellular distribution of certain cytokine receptors has hampered attempts to define the physiologically important cell-specific functions of cytokines in vivo. Herein, we report the generation of transgenic mice that express a dominant-negative IFN gamma receptor alpha chain mutant under the control of either the human lysozyme promoter or the murine lck proximal promoter, which display tissue-specific unresponsiveness in the macrophage or T cell compartments, respectively, to the pleiotropic cytokine, IFN gamma. We utilize these mice to identify previously undefined cellular targets of IFN gamma action in the development of a murine antimicrobial response and the mixed lymphocyte reaction. Moreover, we identify the macrophage as a critical responsive cell in manifesting the effects of IFN gamma in regulating CD4+ T helper subset development. These studies thus represent a novel approach to studying the cell-specific actions of an endogenously produced pleiotropic cytokine in vivo.

Dendritic cells produce IL-12 and direct the development of Th1 cells from naive CD4+ T cells.

Dendritic cells are APCs that are unique in their potency to stimulate proliferation of primary Ag-specific responses in vitro and in vivo. In this study, we demonstrate that dendritic cells can produce IL-12, a dominant cytokine involved in the development of IFN-gamma-producing T cells. This finding resulted from our observations that dendritic cell-induced Th1 development from total CD4+ T cells upon neutralization of endogenous levels of IL-4 was IL-12-dependent. Furthermore, we demonstrate that dendritic cells can induce the development of Th1 cells from Ag-specific naive LECAM-1bright CD4+ T cells obtained from alpha beta-TCR transgenic mice, provided that CD4+ LECAM-1dull T cells, which produce significant levels of IL-4, are not present in the primary cultures. Production of IL-12 by dendritic cells was confirmed by positive immunofluoresence staining with Abs specific for the inducible IL-12 p40 subunit. This suggests that in addition to inducing proliferation and clonal expansion of naive T cells, dendritic cells, by their production of IL-12, play a direct role in the development of IFN-gamma-producing cells that are important for cell-mediated immune responses.

T cell genetic background determines default T helper phenotype development in vitro.

A host's ability to resist certain pathogens such as Leishmania major can depend upon the phenotype of T helper (Th) subset that develops. Different murine genetic backgrounds are known to significantly alter the direction of Th subset development, although the cellular basis of this influence is poorly understood. To examine the basis of this effect we used an in vitro alpha/beta-T cell receptor (TCR) transgenic system for analysis of Th phenotype development. To control for TCR usage, we derived the DO11.10 alpha/beta-TCR transgene in several genetic backgrounds. Our findings suggest that the effects of genetic background on Th phenotype development reside within the T cell, and not the antigen-presenting cell compartment. Transgenic T cells from both the B10.D2 and BALB/c backgrounds showed development toward either the Th1 or Th2 phenotype under the strong directing influence of interleukin (IL) 12 and IL4, respectively. However, when T cells were activated in vitro under neutral conditions in which exogenous cytokines were not added, B10.D2-derived T cells acquired a significantly stronger Th1 phenotype than T cells from the BALB/c background, correspondent with in vivo Th responses to Leishmania in these strains. Importantly, these cytokine differences resulted in distinct functional properties, because B10.D2- but not BALB/c-derived T cells could induce macrophage production of nitric oxide, an important antimicrobial factor. Thus, the genetically determined default Th phenotype development observed in vitro may correspond to in vivo Th subset responses for pathogens such as Leishmania which do not initiate strong Th phenotype-directing signals.