Robotic colorectal resection in combination with a multimodal enhanced recovery program - results of the first 100 cases.

PURPOSE: In Germany, colorectal robot-assisted surgery (RAS) has found its way and is currently used as primary technique in colorectal resections at our clinic. We investigated whether RAS can be extensively combined with enhanced recovery after surgery (ERAS®) in a large prospective patient group. METHODS: Using the DaVinci Xi surgical robot, all colorectal RAS from 09/2020 to 01/2022 were incorporated into our ERAS® program. Perioperative data were prospectively recorded using a data documentation system. The extent of resection, duration of the operation, intraoperative blood loss, conversion rate, and postoperative short-term results were analyzed. We documented the postoperative duration of Intermediate Care Unit (IMC) stay and major and minor complications according to the Clavien-Dindo classification, anastomotic leak rate, reoperation rate, hospital-stay length, and ERAS® guideline adherence. RESULTS: One hundred patients (65 colon and 35 rectal resections) were included (median age: 69 years). The median durations of surgery were 167 min (colon resection) and 246 min (rectal resection). Postoperatively, four patients were IMC-treated (median stay: 1 day). In 92.5% of the colon and 88.6% of the rectum resections, no or minor complications occurred postoperatively. The anastomotic leak rate was 3.1% in colon and 5.7% in rectal resection. The reoperation rate was 7.7% (colon resection) and 11.4% (rectal resection). The hospital stay length was 5 days (colon resection) and 6.5 days (rectal resection). The ERAS® guideline adherence rate was 88% (colon resection) and 82.6% (rectal resection). CONCLUSION: Patient perioperative therapy per the multimodal ERAS® concept is possible without any problems in colorectal RAS, leading to low morbidity and short hospital stays.

Reconstruction, oversewing, or taking the anastomosis down - which surgical intervention is most potent in the treatment of anastomotic leaks following colorectal resections?

PURPOSE: Anastomotic leak (AL) following colorectal resections can be treated interventionally. However, most cases require surgical intervention. Thus, several surgical techniques are available, which intend to affect the further course positively. The aim of this retrospective analysis is to determine which surgical technique proves to have the biggest potential in reducing the morbidity and mortality as well as to minimize the need of re-interventions after AL. METHODS: All patients with a history of AL following colorectal resection between 2008 and 2020 were analyzed. Patient's outcomes following surgical treatment of AL, including morbidity and mortality, clinical and para-clinical (laboratory examinations, ultrasound, and CT-scan) detection of AL recurrence, re-intervention rate, and the length of hospital stay were documented and correlated with the surgical technique used (e.g. simply over-sewing the AL, over-sewing the AL with the construction of a protective ileostomy, resection and reconstruction of the anastomosis, peritoneal lavage and transanal drainage, or taking the anastomosis down and constructing an end stoma). RESULTS: A total of 2,724 colorectal resections were documented. Grade C AL occurred in 92 (4.4% AL occurrence-rate) and 31 (7.2% AL occurrence-rate) cases following colon and rectal resections, respectively. The anastomosis was not preservable in 52 and 17 cases following colon and rectal resections, respectively. Therefore, the anastomosis had been taken down and an end-stoma had been constructed. Over-sewing the AL with the construction of a protective ileostomy had the highest anastomosis preservation rate (14 of 18 cases) and lowest re-intervention rate (mean value of 1.5 re-interventions) following colon and rectal resections (7 of 9 cases; mean value, 1.5 re-interventions). CONCLUSION: In cases where an AL is preservable, over-sewing the anastomosis and constructing a protective ileostomy has the greatest potential for positive short-term outcomes following colorectal resections.

Necessity of routine perioperative epidural catheter placement in laparoscopic colorectal resections: a retrospective data analysis.

PURPOSE: Whether epidural anesthesia leads to further improvement in the postoperative course of colorectal procedures is under discussion. The aim of this study was to evaluate the effects of minimally invasive colorectal oncological interventions without epidural anesthesia (EDA). METHODS: This retrospective data analysis included the clinical data of all patients who underwent minimally invasive oncological colorectal resection at our clinic between January 2013 and April 2019. Of 385 patients who met the inclusion criteria, 183 (group I; 47.5% of 385) received EDA, and 202 (group II; 52.5% of 385) received transversus abdominis plane block instead. The relevant target parameters were evaluated and compared between the groups. The postoperative complications were graded according to the Clavien-Dindo classification. RESULTS: The patients in group I (n=183; women, 77; men, 106; age 66.8 years) were younger (p=0.0035), received a urinary catheter more often (99.5% versus [vs.] 28.2% p<0.001), required longer, more frequent arterenol treatment (1.1 vs. 0.6 days; p<0.001), and had a longer intermediate care unit stay than those in group II (2.8 vs. 1.1 days; p<0.001). Postoperative pain levels were not significantly different between the groups (p=0.078). The patients in group I were able to ambulate later than those in group II (4 vs. 2 days; p<0.001). The difference in the postoperative day of the first defecation was not significant between the groups (p=0.236). The incidence of postoperative complications such as bleeding (p=0.396), anastomotic leaks (p=0.113), and wound infections (p=0.641) did not differ between the groups. The patients in group I had significantly longer hospital stays than those in group II (12.2 vs. 9.4 days; p<0.001). CONCLUSION: EDA can be safely omitted from elective minimally invasive colorectal resections, and its omission is not accompanied by any relevant disadvantages to the patient.

Laparoscopic rectal resection without epidural catheters-does it work?

PURPOSE: Placement of an epidural catheter (EC) in colorectal resections is still recommended as a valid measure to achieve a low level of pain. However, EC is associated with increased invasiveness and with an increased risk of bladder emptying disorders and a decrease in blood pressure, which all relate to delayed mobilization. Preliminary data shows that EC placement may not be necessary for laparoscopic colon resections. The aim of this prospective study was to investigate how the omission of EC placement influences short-term postoperative outcomes in laparoscopic rectal resections. METHODS: All laparoscopic rectal resections occurring between 2013 and 2020 were prospectively examined. Resections from January 2013 to February 2018 (group A) were compared with resections from March 2018 to December 2020 (group B; after the internal change of the perioperative pain regime). In addition to EC placement, the other target parameters of our study were urinary catheter placement during the inpatient stay, postoperative pain > 3 days on a numerical rating scale (NRS), mobilization in the first 5 postoperative days, time until the first postoperative bowel movement, postoperative complications according to Clavien-Dindo, intermediate care unit stay (IMC stay) in days, and hospital length of stay in days. RESULTS: In the entire study period, 221 laparoscopic rectal resections were performed: 122 in group A and 99 resections in group B. The frequency of EC placement and urinary catheter placement, postoperative IMC stay, and hospital length of stay was significantly lower in group B (p < 0.05). The postoperative mobilization of patients in group B was possible more quickly. There were no differences in the level of pain, time until the first postoperative bowel movement, and postoperative complications according to Clavien-Dindo. CONCLUSION: Omission of EC placement in laparoscopic rectal resections led to faster mobilization, a shorter IMC stay, and a shorter hospital stay without increasing the pain level. Postoperative complications did not change when an EC was not placed. Therefore, routine EC placement in laparoscopic rectal resections is unnecessary.

Necessity of subcutaneous suction drains in ileostomy reversal (DRASTAR)-a randomized, controlled bi-centered trial.

PURPOSE: Data regarding length of hospital stay of patients undergoing ileostomy reversal are very heterogeneous. There are many factors that may have an influence on the length of postoperative hospital stay, such as postoperative wound infections. One potential strategy to reduce their incidence and to decrease hospital stay is to insert subcutaneous suction drains. The purpose of this study was to examine the influence of the insertion of subcutaneous suction drains on hospital stay and postoperative wound infections in ileostomy reversal. Risk factors for postoperative wound infection were determined. METHODS: This is a randomized controlled two-center non-inferiority trial with two parallel groups. The total length of hospital stay as primary endpoint and the occurrence of a surgical site infection, the colonization of the abdominal wall with bacteria, and the occurrence of hematomas/seromas as secondary endpoints were monitored. RESULTS: One hundred eighteen patients with elective ileostomy reversal were included. Fifty-nine patients were randomly assigned to insertion of a subcutaneous suction drain, and 59 patients were randomly assigned to receive no drain. After 3 months of follow-up, 50 patients in the group with drain and 53 patients in the group without drain could be analyzed. Median total length of hospital stay was 8 days in the SD group and 9 days in the group without SD (p = 0.17). Fourteen percent of patients with SD and 17 % without SD developed SSI, p = 0.68. Multivariate analysis revealed anemia (p < 0.01), intraoperative bowel perforation (p = 0.02) and resident (p = 0.04) or fellow (p = 0.048) performing the operation as risk factors for SSI. CONCLUSIONS: This trial shows that the omission of subcutaneous suction drains is not inferior to the use of subcutaneous suction drains after ileostomy reversal in terms of length of hospital stay, surgical site infections, and hematomas/seromas.

Therapy of complicated Crohn's disease during pregnancy--an interdisciplinary challenge.

BACKGROUND: Severe courses of Crohn's disease (CD) during pregnancy are rare. However, if occurring, the risk of miscarriage and low birth weight is increased. At present, only limited data is available on the treatment of CD during pregnancy. In particular, there are no standard guidelines for surgical therapy. Nevertheless, surgery is often unavoidable if complications during the course of the disease arise. PURPOSE: This study provides a critical overview of conventional and interventional treatment options for CD complications during pregnancy and analyses the surgical experience gained thus far. For illustrative purposes, clinical cases of three young women with a severe clinical course during pregnancy are presented. METHODS: After treatment-refractory for conservative and interventional measures, surgery remained as the only treatment option. In all cases, a split stoma was created after resection to avoid anastomotic leaks that would endanger the lives of mother and child. The postoperative course of all three patients was uneventful, and pregnancy remained intact until delivery. No further CD specific medication was required before birth. CONCLUSIONS: The management of CD patients during pregnancy requires close interdisciplinary co-operation between gastroenterologists, obstetricians, anaesthetists and visceral surgeons. For the protection of mother and child treatment should thus be delivered in a specialised centre. This article demonstrates the advantages of surgical therapy by focusing on alleviating CD complaints and preventing postoperative complications.

[Five case reports on granulosa cell tumors in cattle with practical information on diagnosis and possible progression]. / Fünf Fallberichte zum Granulosazelltumor beim Rind mit praktischen Hinweisen zur Diagnostik und zum möglichen Verlauf.

INTRODUCTION: Five cases of ovarian tumors (granulosa cell tumors) in cattle are presented from the patient load of the Vetsuisse University of Zurich and Bern. The aim of this work was to demonstrate the variable development of the illness and to indicate diagnostic and therapeutic possibilities to the practicing veterinarians. Case 1 shows bilateral appearance and the development of malignancy and metastases. The main symptoms in case 2 were the development of the mammary gland in a juvenile animal and the behavior modification due to a hormonal imbalance. The cases 3, 4 and 5 underwent surgery, case 4 restarted reproductive activity resulting in five subsequent pregnancies. The initial presumption is a result of a gynecological including ultrasonographic examination and can be verified by the analysis of Müllerian Inhibiting Hormone in serum. The decision to perform surgery should be done rapidly, as normal fertility can be achieved if the tumor is located unilaterally. Tumor growth and potential malignancy can provoke fatal health issues and also make it impossible to use meat of these animals for consumption.

INTRODUCTION: Cinq cas de tumeurs ovariennes (tumeurs des cellules de la granulosa) chez les bovins sont présentés à partir de la patientèle de l'Université Vetsuisse de Zurich et de Berne. Le but de ce travail était de montrer l'évolution variable de la maladie et d'indiquer les possibilités diagnostiques et thérapeutiques aux vétérinaires praticiens. Le cas 1 montre la possibilité d'une apparition bilatérale avec développement d'une tumeur maligne et de métastases. Les principaux symptômes du cas 2 étaient le développement de la glande mammaire chez un animal juvénile et la modification du comportement due à un déséquilibre hormonal. Les cas 3, 4 et 5 ont subi une intervention chirurgicale, le cas 4 a repris une activité de reproduction avec cinq gestations ultérieures. La présomption initiale résulte d'un examen gynécologique et peut être vérifiée par l'analyse de l'hormone anti-müllérienne dans le sérum. La décision d'opérer doit être prise rapidement, car une intervention chirurgicale pour enlever l'ovaire atteint peut, dans l'idéal, permettre une reprise de l'activité de reproduction. La croissance de la tumeur et la malignité potentielle peuvent provoquer des problèmes de santé fatals et rendre la viande de ces animaux impropre à la consommation.

Suitability of a virtual reality simulator for laparoscopic skills assessment in a surgical training course.

BACKGROUND: Virtual reality simulators (VRS) can acquire specific performance parameters for laparoscopic surgery. The aim of this study was to evaluate the suitability of a VRS for the assessment in a surgical skills course. MATERIALS AND METHODS: One hundred five attendees of a 7-day surgical skills course were tested with a VRS at the beginning (T1) and at the end (T2) of the course. Two standard VRS tasks (lifting and grasping (LG) and fine dissection (FD)) with two scores and 21 individual parameters were used. VRS performance was correlated to laparoscopic experience and experience in playing video games in order to assess the influence of preexisting skills. RESULTS: The participants improved significantly in both scores and in 19/21 VRS parameters between T1 and T2. Laparoscopic experts were significantly better than novices only for the parameter tissue damage on T1 in LG (41.4 %, P < 0.001). Gamers were significantly better than non-gamers in all manual parameters on T1 in LG. Both groups of laparoscopic experience as well as non-gamers improved between T1 and T2 in LG for most parameters, while gamers only improved for tissue damage. CONCLUSIONS: The VRS was able to assess the gain in surgical performance during the course in general. However, laparoscopic experience and video game experience strongly influenced the results. Laparoscopic experience was correlated to the parameter tissue damage, whereas video game experience was correlated to manual parameters. This knowledge can be used to build adequate scoring systems for VRS and to design tasks that target specific course skills.

Reduction of wound infections in laparoscopic-assisted colorectal resections by plastic wound ring drapes (REDWIL)?--A randomized controlled trial.

INTRODUCTION: Surgical site infections (SSIs) are frequent complications in colorectal surgery and may lead to burst abdomen, incisional hernia, and increased perioperative costs. Plastic wound ring drapes (RD) were introduced some decades ago to protect the abdominal wound from bacteria and reduce SSIs. There have been no controlled trials examining the benefit of RD in laparoscopic colorectal surgery. The Reduction of wound infections in laparoscopic assisted colorectal resections by plastic wound ring drapes (REDWIL) trial was thus designed to assess their effectiveness in preventing SSIs after elective laparoscopic colorectal resections. MATERIALS/METHODS: REDWIL is a randomized controlled monocenter trial with two parallel groups (experimental group with RD and control group without RD). Patients undergoing elective laparoscopic colorectal resection were included. The primary endpoint was SSIs. Secondary outcomes were colonization of the abdominal wall with bacteria, reoperations/readmissions, early/late postoperative complications, and cost of hospital stay. The duration of follow-up was 6 months. RESULTS: Between January 2008 and October 2010, 109 patients were randomly assigned to the experimental or control group (with or without RD). Forty-six patients in the RD group and 47 patients in the control group completed follow-up. SSIs developed in ten patients with RD (21.7 %) and six patients without RD (12.8 %) (p = 0.28). An intraoperative swab taken from the abdominal wall was positive in 66.7 % of patients with RD and 57.5 % without RD (p = 0.46). The number of species cultured within one swab was significantly higher in those without RD (p = 0.03). The median total inpatient costs including emergency readmissions were 3,402 ± 4,038 in the RD group and 3,563 ± 1,735 in the control group (p = 0.869). CONCLUSIONS: RD do not reduce the rate of SSIs in laparoscopic colorectal surgery. The inpatient costs are similar with and without RD.

[Simulation training in surgical education - application of virtual reality laparoscopic simulators in a surgical skills course]. / Simulationstraining in der Weiterbildung - Einsatz von virtuellen laparoskopischen Operationssimulatoren in einem chirurgischen Trainingskurs.

BACKGROUND: Training and simulation are gaining importance in surgical education. Today, virtual reality surgery simulators provide sophisticated laparoscopic training scenarios and offer detailed assessment methods. This also makes simulators interesting for the application in surgical skills courses. The aim of the current study was to assess the suitability of a virtual surgery simulator for training and assessment in an established surgical training course. MATERIALS AND METHODS: The study was conducted during the annual "Practical Course for Visceral Surgery" (Warnemuende, Germany). 36 of 108 course participants were assigned at random for the study. Training was conducted in 15 sessions over 5 days with 4 identical virtual surgery simulators (LapSim) and 2 standardised training tasks. The simulator measured 16 individual parameters and calculated 2 scores. Questionnaires were used to assess the test persons' laparoscopic experience, their training situation and the acceptance of the simulator training. Data were analysed with non-parametric tests. A subgroup analysis for laparoscopic experience was conducted in order to assess the simulator's construct validity and assessment capabilities. RESULTS: Median age was 32 (27 - 41) years; median professional experience was 3 (1 - 11) years. Typical laparoscopic learning curves with initial significant improvements and a subsequent plateau phase were measured over 5 days. The individual training sessions exhibited a rhythmic variability in the training results. A shorter night's sleep led to a marked drop in performance. The participants' different experience levels could clearly be discriminated ( ≤ 20 vs. > 20 laparoscopic operations; p ≤ 0.001). The questionnaire showed that the majority of the participants had limited training opportunities in their hospitals. The simulator training was very well accepted. However, the participants severely misjudged the real costs of the simulators that were used. CONCLUSIONS: The learning curve on the simulator was successfully mastered during the course. Construct validity could be demonstrated within the course setting. The simulator's assessment system can be of value for the assessment of laparoscopic training performance within surgical skills courses. Acceptance of the simulator training is high. However, simulators are currently too expensive to be used within a large training course.

[First 18 months as certified ERAS® center for colorectal cancer : Lessons learned and results of the first 261 patients]. / Achtzehn Monate als zertifiziertes ERAS®-Zentrum für kolorektale Resektionen : "Lessons learned" und Ergebnisse der ersten 261 Patienten.

BACKGROUND: Enhanced recovery after surgery (ERAS®) describes a multimodal, interdisciplinary and interprofessional treatment concept that optimizes the postoperative convalescence of the patient through the use of evidence-based measures. GOAL OF THE WORK: The aim of this article is to present the experiences of our center certified by the ERAS® Society for colorectal resections 18 months after successful implementation. MATERIAL AND METHODS: Since the beginning of the certification 261 patients have been treated in our clinic according to the specifications of the ERAS® concept. As a comparison group the last 50 patients prior to implementation were evaluated in terms of compliance with ERAS® requirements, length of hospital stay and readmission rate, the need for care in an intensive or intermediate care ward, the number of necessary reoperations and the complication rate. RESULTS: Compliance increased from 39.3% preERAS® to 81.1% after ERAS® implementation (p < 0.001). At the same time the length of stay of ERAS® patients was reduced from 7 days to 5 days (p = 0.001). While the rate of surgical complications was the same between the two groups (p = 0.236), nonsurgical complications occurred significantly less frequently in the ERAS® cohort (p = 0.018). DISCUSSION: There are well-known stumbling blocks in implementing and maintaining an ERAS® concept; however, it is worthwhile for the patient to circumnavigate this and establish ERAS® as the standard treatment path.

Fludarabine-induced immunosuppression is associated with inhibition of STAT1 signaling.

Fludarabine is a nucleoside analog used in the treatment of hematologic malignancies that can induce severe and prolonged immunosuppression. Although it can be incorporated into the DNA of dividing cells, fludarabine is also a potent inhibitor of cells with a low growth fraction, thus it must have other mechanisms of action. STAT1, which is activated in response to many lymphocyte-activating cytokines including the interferons, is essential for cell-mediated immunity, as the absence of this protein is associated with prominent defects in the ability to control viral infections. Here we show that fludarabine, but not the immunosuppressant cyclosporine A, inhibits the cytokine-induced activation of STAT1 and STAT1-dependent gene transcription in normal resting or activated lymphocytes. Fludarabine caused a specific depletion of STAT1 protein (and mRNA) but not of other STATs. This loss of STAT1 was also seen in cells from patients treated with fludarabine in vivo. Brief exposure to fludarabine led to a sustained loss of STAT1, analogous to the prolonged period of immunosuppression induced by exposure to the drug in vivo. Thus, STAT1 may be a useful target in the development of new immunosuppressive and antineoplastic agents.

Identification and cloning of a prethymic precursor T lymphocyte from a population of common acute lymphoblastic leukemia antigen (CALLA)-positive fetal bone marrow cells.

We have cloned common acute lymphoblastic leukemia (CALLA)-positive cells from human fetal bone marrow containing less than 1 in 10,000 E-RFC in round-bottomed microtiter wells (one cell per well) using the autocloning unit of an EPICS-V cell sorter. Expansion of such cells (with IL-2 and heavily irradiated autologous thymocytes as feeder cells) resulted in growth in 6-14% of the wells (mean, 11%) with cells with mature T lymphocyte phenotype. Two-color fluorescence analysis of outgrowing cultures furthermore ascertained that these cells had differentiated through a phase of simultaneous expression of T4 and T8 antigens and at the same time expression of the thymocyte-associated T6 antigens. Thus, given the fact that 10-20% of T cell acute lymphoblastic leukemia (T-ALLs) are CALLA+, we have been able to identify a human prethymic T lymphocyte population that might be the normal counterpart of precursor cell to the CALLA+ T-ALL cell.

CD2 regulates responsiveness of activated T cells to interleukin 12.

Interleukin (IL) 12 is a 70-kD heterodimeric cytokine produced by antigen-presenting cells (APCs) such as macrophages in response to infectious pathogens and interferon (IFN) gamma. The varied immunomodulatory effects of IL-12 include the stimulation of proliferation and IFN-gamma production by T cells, and it also has a central role in the development of the T helper cell type 1 immune phenotype. We undertook the production of antibodies capable of modulating the response of T cells to IL-12, and in the process we discovered two antibodies that inhibited the ability of IL-12 to stimulate T cell proliferation. In this report, we demonstrate that these anti-bodies recognize CD2, and we show how antibodies directed toward either the adhesion domain of CD2 or its ligand, CD58, specifically inhibit IL-12 induced proliferation and IFN-gamma production by phytohemagglutinin-activated T cells, leaving the response to IL-12 unaffected. A three-to fourfold reduction in proliferation and IFN-gamma production was observed at IL-12 concentrations as high as 1 nM, with complete inhibition occurring at < or = 1 pM. This novel effect is not directly mediated at the level of the IL-12 receptor, as shown by the inability of these antibodies to block IL-12 binding to activated T cells. Furthermore, by using activating pairs of CD2 antibodies, we show that CD2 stimulation strongly synergizes with IL-12, even at 0.1 pM, in inducing both T cell proliferation and IFN-gamma production. Cytolytic T lymphocyte-associated antigen 4-immunoglobulin-mediated inhibition of the B7/CD28 interaction did not affect the T cell response to either IL-12 or IL-2, but the removal of APCs selectively diminished the proliferative response to IL-12. Based on this data, we hypothesize that CD2 has a central role in an IL-12/IFN-gamma positive feedback loop between T cell and APC, providing the key functional link via a CD2/CD58 interaction that controls T cell responsiveness to IL-12. This model provides a basis for future investigations aimed at defining the signaling mechanisms that mediate this cytokine-specific regulatory effect of CD2, and it offers insight into how a cytokine receptor and distinct adhesion molecule can interact to modulate responsiveness to that cytokine. In addition, it underscores the possibility that the clinical potential of an immunomodulatory drug like IL-12 may be governed by the presence or absence of specific costimulation.

A subset of natural killer cells in peripheral blood displays a mature T cell phenotype.

Normal human PBMC were analyzed for the presence of cells expressing both T3 and NKH1 antigens, using direct two-color immunofluorescence. In six individuals, NKH1+T3+ cells were found to represent 2.5% of PBMC and 24% of the total number of NKH1+ cells. Purified NKH1+T3+ cells were shown to have the typical morphology of large granular lymphocytes (LGL). NKH1+T3+ cells also exhibited spontaneous cytotoxicity against K562 target cells and this lytic activity could be inhibited by anti-T3 mAb. Similar results were obtained with NKH1+T3+ cells cultured in vitro in lymphocyte-conditioned medium. Taken together, these results indicate that NKH1+T3+ cells represent a unique population of NK-active cells in normal peripheral blood. Although these cells exhibit LGL morphology and NK activity, this appears to be mediated through a functional T cell-like receptor for target antigen.

Functional consequences of interleukin 2 receptor expression on resting human lymphocytes. Identification of a novel natural killer cell subset with high affinity receptors.

In this study, we have used radiolabeled IL-2 binding assays, Northern blot analysis, immunofluorescent flow cytometry and cell sorting, as well as proliferation and cytotoxicity assays to perform an extensive phenotypic and functional characterization of the IL-2 receptor in normal resting human peripheral blood lymphocytes. Our results indicate that almost all T cells (greater than 98%) express neither the high affinity IL-2 receptor nor the functional intermediate affinity p75 chain of the IL-2 receptor without prior activation. In contrast, most NK cells constitutively express the isolated intermediate affinity p75 IL-2 receptor. In addition, a subpopulation of NK cells, distinguished by high density expression of the NKH1 antigen, constitutively express the high affinity IL-2 receptor, in addition to an excess of the isolated intermediate affinity p75 IL-2 receptor. These NKH1bright+ cells exhibit a brisk proliferative response to IL-2, similar to that seen with antigen-activated T cells, yet do so in the absence of any known antigenic stimuli. No other resting peripheral blood lymphocyte population, including CD4+, CD8+, and CD20 cells, exhibits this property. The intermediate affinity p75 IL-2 receptor, as it exists in its isolated form on resting NK cells, does not transduce a growth signal equivalent to that seen in NK cells expressing the high affinity IL-2 receptor, despite doses of IL-2 that are known to fully saturate the isolated p75 chain. This strongly suggests that additional structural or functional components are involved in generating the proliferative response following the binding of IL-2 to the high affinity heterodimeric form of the IL-2 receptor. The constitutive expression of this functional high affinity IL-2 receptor on a small population of resting NK cells provides further evidence in support of a role for these cells in the host's early defense against viral infection or malignant transformation, before the more delayed but specific T cell response.