Race, ethnicity, ancestry, and aspects that impact HLA data and matching for transplant.

Race, ethnicity, and ancestry are terms that are often misinterpreted and/or used interchangeably. There is lack of consensus in the scientific literature on the definition of these terms and insufficient guidelines on the proper classification, collection, and application of this data in the scientific community. However, defining groups for human populations is crucial for multiple healthcare applications and clinical research. Some examples impacted by population classification include HLA matching for stem-cell or solid organ transplant, identifying disease associations and/or adverse drug reactions, defining social determinants of health, understanding diverse representation in research studies, and identifying potential biases. This article describes aspects of race, ethnicity and ancestry information that impact the stem-cell or solid organ transplantation field with particular focus on HLA data collected from donors and recipients by donor registries or transplant centers.

Serial Bronchoalveolar Lavage Fluid Aspergillus Galactomannan and Treatment Response in Invasive Pulmonary Aspergillosis.

We studied patients diagnosed with aspergillosis based on positive bronchoalveolar lavage (BAL) Aspergillus galactomannan (GM) who had follow-up BAL sampling within 180 days. GM trend and clinical outcome were concordant in only 60% (30/50). While useful for the initial diagnosis, BAL GM trending does not always correlate with treatment response.

Updates in Skin Cancer in Transplant Recipients and Immunosuppressed Patients: Review of the 2022-2023 Scientific Symposium of the International Immunosuppression and Transplant Skin Cancer Collaborative.

The International Immunosuppression and Transplant Skin Cancer Collaborative (ITSCC) and its European counterpart, Skin Care in Organ Transplant Patients-Europe (SCOPE) are comprised of physicians, surgeons, and scientist who perform integrative collaborative research focused on cutaneous malignancies that arise in solid organ transplant recipients (SOTR) and patients with other forms of long-term immunosuppression. In October 2022, ITSCC held its biennial 4-day scientific symposium in Essex, Massachusetts. This meeting was attended by members of both ITSCC and SCOPE and consisted of specialists including Mohs micrographic and dermatologic oncology surgeons, medical dermatologists, transplant dermatologists, transplant surgeons, and transplant physicians. During this symposium scientific workshop groups focusing on consensus standards for case reporting of retrospective series for invasive squamous cell carcinoma (SCC), defining immunosuppressed patient status for cohort reporting, development of multi-institutional registry for reporting rare tumors, and development of a KERACON clinical trial of interventions after a SOTRs' first cutaneous SCC were developed. The majority of the symposium focused on presentation of the most up to date research in cutaneous malignancy in SOTR and immunosuppressed patients with specific focus on chemoprevention, immunosuppression regimens, immunotherapy in SOTRs, spatial transcriptomics, and the development of cutaneous tumor registries. Here, we present a summary of the most impactful scientific updates presented at the 2022 ITSCC symposium.

Clinical Outcomes of Renal Transplant Recipients Undergoing Percutaneous Coronary Intervention.

BACKGROUND: Clinical outcomes of patients with renal transplant (RT) undergoing percutaneous coronary intervention (PCI) remain poorly elucidated. METHOD: Between 2014 and 2021, data were analysed for the following three groups of patients undergoing PCI enrolled in a multicentre Australian registry: (1) RT recipients (n=226), (2) patients on dialysis (n=992), and (3) chronic kidney disease (CKD) patients (estimated glomerular filtration rate [eGFR], 30‒60 mL/min per 1.73 m2) without previous RT (n=15,534). Primary outcome was 30-day major adverse cardiac and cerebrovascular events (MACCEs)-composite of mortality, myocardial infarction, stent thrombosis, target vessel revascularisation, and stroke. RESULTS: RT recipients were younger than dialysis and patients with CKD (61±10 vs 68±12 vs 78±8.2 years, p<0.001). Patients with RT less frequently had severe left ventricular dysfunction compared with dialysis and CKD groups (6.7% vs 14% and 8.5%); however more, often presented with acute coronary syndrome (58% vs 52% and 48%), especially STEMI (all p<0.001). Patients with RT and CKD had lower rates of 30-day MACCE (4.4% and 6.8% vs 11.6%, p<0.001) than the dialysis group. Three-year survival was similar between RT and CKD groups, however was lower in the dialysis group (80% and 83% vs 60%, p<0.001). After adjustment, dialysis was an independent predictor of 30-day MACCE (odds ratio [OR] 1.90, 95% confidence interval [CI] 1.44‒2.50, p<0.001), however RT was not (OR 0.91, CI 0.42‒1.96, p=0.802). Both RT (hazard ratio [HR] 2.07, CI 1.46‒2.95, p<0.001) and dialysis (HR 1.35, CI 1.02‒1.80, p=0.036) heightened the hazard of long-term mortality. CONCLUSIONS: RT recipients have more favourable clinical outcomes following PCI compared with patients on dialysis. However, despite having similar short-term outcomes to patients with CKD, the hazard of long-term mortality is significantly greater for RT recipients.

Primary Liver Transplant in Biliary Atresia: The Case for and Against.

The role of liver transplantation as a primary procedure in biliary atresia has been argued over for at least 40 years, indeed since the coming of age of safe liver transplantation during the 1980s. Yet, it is not a common option in most series (usually ≤5%) and typically reserved for those with late presentations (arguably >100 days) with established cirrhosis. This review presents the pros and cons of primary liver transplant. The pros are based upon the observation that at best a Kasai portoenterostomy (KPE) is simply palliative in most, and at worse has no effect whatsoever on restoration of bile flow and is therefore pointless. Set against this are the cons: there is a dearth of prognostic tests (clinical, biochemical, or histological) at the time of presentation which may predict inevitable failure; the possibility of long-term native liver survival to adulthood in a proportion (albeit a minority); and the implied increased need for donor organs suitable for infants - a stressor for an already overstressed system. Improving results from KPE in terms of increasing the proportions clearing their jaundice and minimizing the effects of chronic liver fibrosis and cirrhosis would surely limit the siren calls for primary transplants but the key must be better discrimination at presentation with the use of biomarkers (circulatory or histological, individually or together) to enable better decision making.

Implementation Science to Achieve Equity in Heart Failure Care: A Scientific Statement From the American Heart Association.

Guideline-directed medical therapies and guideline-directed nonpharmacological therapies improve quality of life and survival in patients with heart failure (HF), but eligible patients, particularly women and individuals from underrepresented racial and ethnic groups, are often not treated with these therapies. Implementation science uses evidence-based theories and frameworks to identify strategies that facilitate uptake of evidence to improve health. In this scientific statement, we provide an overview of implementation trials in HF, assess their use of conceptual frameworks and health equity principles, and provide pragmatic guidance for equity in HF. Overall, behavioral nudges, multidisciplinary care, and digital health strategies increased uptake of therapies in HF effectively but did not include equity goals. Few HF studies focused on achieving equity in HF by engaging stakeholders, quantifying barriers and facilitators to HF therapies, developing strategies for equity informed by theory or frameworks, evaluating implementation measures for equity, and titrating strategies for equity. Among these HF equity studies, feasibility was established in using various educational strategies to promote organizational change and equitable care. A couple include ongoing randomized controlled pragmatic trials for HF equity. There is great need for additional HF implementation trials designed to promote delivery of equitable guideline-directed therapy.

Distinct effects of racial and socioeconomic disparities on biliary atresia diagnosis and outcome.

OBJECTIVES: To identify and distinguish between racial and socioeconomic disparities in age at hepatology care, diagnosis, access to surgical therapy, and liver transplant-free survival in patients with biliary atresia (BA). METHODS: Single-center retrospective cohort study of 69 BA patients from 2010 to 2021. Patients were grouped into White and non-White cohorts. The socioeconomic milieu was analyzed utilizing neighborhood deprivation index, a census tract-based calculation of six socioeconomic variables. The primary outcomes of this study were timing of the first hepatology encounter, surgical treatment with hepatic portoenterostomy (HPE), and survival with native liver (SNL) at 2 years. RESULTS: Patients were 55% male and 72% White. White patients were referred at a median of 34 days (interquartile range [IQR]: 17-65) vs. 67 days (IQR: 42-133; p = 0.001) in non-White patients. White infants were more likely to undergo HPE (42/50 patients; 84%) compared to non-White (10/19; 53%), odds ratio (OR) 4.73 (95% confidence interval: 1.46-15.31; p = 0.01). Independent of race, patients exposed to increased neighborhood-level deprivation were less likely to receive HPE (OR: 0.49, p = 0.04) and achieve SNL (OR: 0.54, p = 0.02). CONCLUSIONS: Racial and socioeconomic disparities are independently associated with timely BA diagnosis, access to surgical treatment, and transplant-free survival. Public health approaches to improve screening for pathologic jaundice in infants of diverse racial backgrounds and to test and implement interventions for socioeconomically at-risk families are needed.

From Blunt Cardiac Injury to Heart Transplant Following Motorcycle Collision.

Traumatic coronary artery occlusion and dissection is an exceedingly rare complication of blunt cardiac injury (BCI), though it has been previously noted in a number of case reports. However, it can also lead to heart transplant, which to our knowledge has not been previously described in the literature. We present a case of a healthy 24-year-old man without significant past medical history who was in a motorcycle accident, resulting in sternal fracture and BCI. He was ultimately found to have thrombotic occlusion and dissection of his left anterior descending artery (LAD), requiring mechanical thrombectomy and drug-eluting stent, as well as subsequent hospitalizations and operations due to various complications. It was suspected that he went into ventricular fibrillation and had a second motorcycle collision, resulting in cardiogenic shock. Ultimately, his progression of ischemic cardiomyopathy and mitral regurgitation led to the need for heart transplant. Blunt cardiac injury with myocardial contusion has such a broad range of pathologies. It is essential that patients with these injury patterns raise a high level of suspicion for BCI and are followed closely with appropriate diagnostic testing and rapid intervention for best possible outcomes.

Biliary atresia in Uganda: Current ethical challenges and advancement of public policy.

Biliary atresia is a progressive cholangiopathy in neonates, which often results in liver failure. In high-income countries, initial treatment requires prompt diagnosis followed by Kasai portoenterostomy. For those with a late diagnosis, or those in whom Kasai portoenterostomy fails, liver transplantation is the only lifesaving treatment. Unfortunately, in low- and middle-income countries, timely diagnosis is a challenge and liver transplantation is rarely accessible. Here, we discuss the ethical dilemmas surrounding treatment of babies with biliary atresia in Uganda. Issues that require careful consideration include: risk of catastrophic health expenditure to families, ethical dilemmas of transplant tourism, medical risks of maintaining the transplant in a low-resourced health system, and difficult decisions encountered by the surgeon caring for these patients. Four distinct models of the patient-physician relationship are applied to biliary atresia in Uganda. These models describe differences in patient and physician roles, and patient values and autonomy. Solid organ transplantation is a rapidly evolving segment of healthcare in Uganda and ongoing policy advancements may shift ethical considerations in the future.

Bone marrow transplantation and bone marrow-derived mesenchymal stem cell therapy in epidermolysis bullosa: A systematic review.

Epidermolysis bullosa (EB) is a genodermatosis that lacks effective treatments and requires supportive care for its severe, life-threatening manifestations. Bone marrow transplantation (BMT) and its derived cells have been suggested to improve clinical symptoms and quality of life. A comprehensive search was conducted for publications evaluating BMT and bone marrow-derived mesenchymal stem cell (BM-MSC) therapy for EB in PubMed/MEDLINE, Google Scholar, and Cochrane databases from inception until June 2023. A total of 55 participants with severe forms of EB had BMT and/or BM-MSCs, with recessive dystrophic EB as the most common EB type; 53 (96.4%) patients had better wound healing, and 3 (5.5%) patients died of sepsis. The most common adverse events reported were graft failure, sepsis, graft-versus-host disease, and renal insufficiency. Allogeneic BMT is a high-risk procedure with possible benefits and adverse events. BM-MSCs revealed favorable outcomes to improve the safety of EB cell-based therapy by minimizing the risk of serious adverse events, reducing blisters, and accelerating wound healing. Further studies are needed to assess the treatment's long-term effects and clarify the risk/benefit ratio of procedure versus conventional therapy.

The use of ferumoxytol for high-resolution vascular imaging and troubleshooting for abdominal allografts.

Ferumoxytol is an ultrasmall superparamagnetic iron oxide which has been used as an off-label intravenous contrast agent for MRI. Unlike gadolinium-based contrast agents, ferumoxytol remains in the intravascular space with a long half-life of 14-21 h. During the first several hours, it acts as a blood-pool agent and has minimal parenchymal enhancement. Studies have shown adequate intravascular signal for up to 72 h after initial contrast bolus. Ferumoxytol has been shown to be safe, even in patients with renal failure. Ferumoxytol has shown promise in a variety of clinical settings. The exquisite resolution enabled by the long intravascular times and lack of background parenchymal enhancement is of particular interest in the vascular imaging of solid organ allografts. Ferumoxytol magnetic resonance angiography (MRA) may identify clinically actionable findings months before ultrasound, CT angiography, or Gadolinium-enhanced MRA. Ferumoxytol MRA is of particular benefit as a troubleshooting tool in the setting of equivocal ultrasound and CT imaging. In the following review, we highlight the use of ferumoxytol for high-resolution MR vascular imaging for abdominal solid organ allografts, with representative cases.

Intraoperative and Postoperative Hemodynamic Predictors of Acute Kidney Injury in Pediatric Heart Transplant Recipients.

Acute kidney injury (AKI) is common after pediatric heart transplantation (HT) and is associated with inferior patient outcomes. Hemodynamic risk factors for pediatric heart transplant recipients who experience AKI are not well described. We performed a retrospective review of 99 pediatric heart transplant patients at Lucile Packard Children's Hospital Stanford from January 1, 2015, to December 31, 2019, in which clinical and demographic characteristics, intraoperative perfusion data, and hemodynamic measurements in the first 48 postoperative hours were analyzed as risk factors for severe AKI (Kidney Disease: Improving Global Outcomes [KDIGO] stage ≥ 2). Univariate analysis was conducted using Fisher's exact test, Chi-square test, and the Wilcoxon rank-sum test, as appropriate. Multivariable analysis was conducted using logistic regression. Thirty-five patients (35%) experienced severe AKI which was associated with lower intraoperative cardiac index ( p = 0.001), higher hematocrit ( p < 0.001), lower body temperature ( p < 0.001), lower renal near-infrared spectroscopy ( p = 0.001), lower postoperative mean arterial blood pressure (MAP: p = 0.001), and higher central venous pressure (CVP; p < 0.001). In multivariable analysis, postoperative CVP >12 mm Hg (odds ratio [OR] = 4.27; 95% confidence interval [CI]: 1.48-12.3, p = 0.007) and MAP <65 mm Hg (OR = 4.9; 95% CI: 1.07-22.5, p = 0.04) were associated with early severe AKI. Children with severe AKI experienced longer ventilator, intensive care, and posttransplant hospital days and inferior survival ( p = 0.01). Lower MAP and higher CVP are associated with severe AKI in pediatric HT recipients. Patients, who experienced AKI, experienced increased intensive care unit (ICU) morbidity and inferior survival. These data may guide the development of perioperative renal protective management strategies to reduce AKI incidence and improve patient outcomes.

Huge mesenchymal hamartoma in a young adult: a case report.

Mesenchymal hamartoma of the liver (MHL) is rare. Less than 50 adult cases have been described. Due to their potential degeneration or recurrence, a complete surgical resection must be performed. We describe a case of a 26-year-old with a palpable solid lesion, which displaced abdominal organs. Percutaneous needle biopsies suggested the diagnosis of MHL. A right hemi-hepatectomy without segment 1 was performed; the post-operative course was uneventful. The mesenchymal component of the tumour was reactive to desmin and smooth muscle actin. Low proliferation index was confirmed (MIB1). Genetic counselling: the sequencing analysis of DICER1 and CDKN1C gene was negative, DNA methylation analysis on the chromosome 11p15 region was normal. After 42 months, there was no recurrence. In conclusion, clinicians should consider MHL in the differential diagnosis. The dimension and the need of radicality impose major liver resections or liver transplantations, which should be performed in referral centres.

Better kidney allograft survival despite higher-risk donor and recipient characteristics between 1995-2014.

BACKGROUND AND HYPOTHESIS: Advances in organ procurement, surgical techniques, immunosuppression regimens and prophylactic antibiotic therapies have dramatically improved short term kidney transplant graft failure. It is unclear how these interventions have affected longer term graft failure. It is hypothesised that graft failure has improved over the last 20 years. METHODS: Data on all first kidney transplants from 1995-2014 were extracted from the Australia and New Zealand Dialysis and Transplant Registry with follow-up as of 31 December, 2021. Primary exposure was transplant era, classified into 5-year intervals. Primary outcome was all-cause 5-year graft failure. Secondary outcomes included all-cause 10-year graft failure and cause-specific graft failure. Kaplan Meier curves and multivariable Cox Proportional Hazards Regression models were used to assess trends in all-cause graft failure. Fine-Gray subdistribution hazard models verified that changes in death rates were not biasing the Cox Proportional Hazards Regression models. Cumulative incidence functions were used to assess temporal trends in cause-specific graft failure. RESULTS: Across 10 871 kidney transplants, there was a shift towards transplanting more recipients aged over 45 years old, with more comorbidities, longer dialysis vintage, body mass index greater than 30 kg/m2 and greater human leukocyte antigen mismatches. Donor age has increased but no clear shift in donor source was observed. Compared to 1995-1999 (reference), the adjusted hazard ratio for 5-year graft failure was 0.78 (95% CI 0.67-0.91), 0.70 (95% CI 0.59-0.83) and 0.60 (95% CI 0.50-0.73) for 2000-2004, 2005-2009, and 2010-2014, respectively. Ten-year graft failure similarly reduced from 0.83 (95% CI 0.74-0.93) for 2000-04 to 0.78 (95% CI 0.68-0.89) for 2010-14, compared to 1995-99. CONCLUSION: Medium and long term all-cause graft failure has improved steadily since 1995-99. Significant reductions in graft failure due to rejection and vascular causes were observed at 5 years, and due to rejection, vascular causes, death and glomerular disease at 10 years.

Mismatched related donor allogeneic haematopoietic cell transplantation compared to other donor types for Ph+ chronic myeloid leukaemia: A retrospective analysis from the Chronic Malignancies Working Party of the EBMT.

Allogeneic haematopoietic cell transplantation (allo-HCT) remains an option for tyrosine kinase inhibitor-resistant chronic myeloid leukaemia (CML) in first chronic phase (CP1) and high-risk patients with advanced disease phases. In this European Society for Blood and Marrow Transplantation (EBMT) registry-based study of 1686 CML patients undergoing first allo-HCT between 2012 and 2019, outcomes were evaluated according to donor type, particularly focusing on mismatched related donors (MMRDs). Median age at allo-HCT was 46 years (IQR 36-55). Disease status was CP1 in 43%, second CP (CP2) or later in 27%, accelerated phase in 12% and blast crisis in 18%. Donor type was matched related (MRD) in 39.2%, MMRD in 8.1%, matched unrelated (MUD) in 40.2%, and mismatched unrelated (MMUD) in 12.6%. In 4 years, overall survival (OS) for MRD, MMRD, MUD and MMUD was 61%, 56%, 63% and 59% (p = 0.21); relapse-free survival (RFS) was 48%, 42%, 52% and 46% (p = 0.03); cumulative incidence of relapse (CIR) was 33%, 37%, 27% and 30% (p = 0.07); non-relapse mortality (NRM) was 19%, 21%, 21% and 24% (p = 0.21); and graft-versus-host disease (GvHD)-free/relapse-free survival (GRFS) was 16%, 18%, 22% and 15% (p = 0.05) respectively. On multivariate analysis, MMRD use associated with longer engraftment times and higher risk of graft failure compared to MRD or MUD. There was no statistical evidence that MMRD use associated with different OS, RFS and incidence of GvHD compared to other donor types.

Surgical Treatment for Primary Lymphedema: A Systematic Review of the Literature.

This is a retrospective review of surgical management for primary lymphedema. Data were extracted from 55 articles from PubMed MEDLINE, Web of Science, SCOPUS, and Cochrane Central Register of Controlled Trials between the database inception and December 2022 to evaluate the outcomes of lymphaticovenous anastomosis (LVA) and vascularized lymph node transfer (VLNT), and outcomes of soft tissue extirpative procedures such as suction-assisted lipectomy (SAL) and extensive soft tissue excision. Data from 485 patients were compiled; these were treated with LVA ( n = 177), VLNT ( n = 82), SAL ( n = 102), and excisional procedures ( n = 124). Improvement of the lower extremity lymphedema index, the quality of life (QoL), and lymphedema symptoms were reported in most studies. LVA and VLNT led to symptomatic relief and improved QoL, reaching up to 90 and 61% average circumference reduction, respectively. Cellulitis reduction was reported in 25 and 40% of LVA and VLNT papers, respectively. The extirpative procedures, used mainly in patients with advanced disease, also led to clinical improvement from the volume reduction, as well as reduced incidence of cellulitis, although with poor cosmetic results; 87.5% of these reports recommended postoperative compression garments. The overall complication rates were 1% for LVA, 13% for VLNT, 11% for SAL, and 46% for extirpative procedures. Altogether, only one paper lacked some kind of improvement. Primary lymphedema is amenable to surgical treatment; the currently performed procedures have effectively improved symptoms and QoL in this population. Complication rates are related to the invasiveness of the chosen procedure.

Post-transplant cyclophosphamide or cell selection in haploidentical allogeneic hematopoietic cell transplantation?

BACKGROUND: One major limitation for broader applicability of allogeneic hematopoietic cell transplantation (allo-HCT) in the past was the lack of HLA-matched histocompatible donors. Preclinical mouse studies using T-cell depleted haploidentical grafts led to an increased interest in the use of ex vivo T-cell depleted (TCD) haploidentical allo-HCT. TCD grafts through negative (T-cell depletion) or positive (CD34+ cell selection) techniques have been investigated to reduce the risk of graft-versus-host disease (GVHD) given the known implications of alloreactive T cells. A more practical approach to deplete alloreactive T cells in vivo using high doses of cyclophosphamide after allografting has proved to be feasible in overcoming the HLA barrier. Such approach has extended allo-HCT feasibility to patients for whom donors could not be found in the past. Nowadays, haploidentical donors represent a common donor source for patients in need of an allo-HCT. The broad application of haploidentical donors became possible by understanding the importance of depleting alloreactive donor T cells to facilitate engraftment and reduce incidence and severity of GVHD. These techniques involve ex vivo graft manipulation or in vivo utilization of pharmacologic agents, notably post-transplant cyclophosphamide (PTCy). DISCUSSION: While acknowledging that no randomized controlled prospective studies have been yet conducted comparing TCD versus PTCy in haploidentical allo-HCT recipients, there are two advantages that would favor the PTCy, namely ease of application and lower cost. However, emerging data on adverse events associated with PTCy including, but not limited to cardiac associated toxicities or increased incidence of post-allograft infections, and others, are important to recognize.

A single-center analysis of lung transplantation outcomes in recipients aged 70 or older.

OBJECTIVES: As life expectancies continue to increase, a greater proportion of older patients will require lung transplants (LTs). However, there are no well-defined age cutoffs for which LT can be performed safely. At our high-volume LT center, we explored outcomes for LT recipients ≥70 years old versus <70 years old. METHODS: This is a retrospective single-center study of survival after LT among older recipients. Data were stratified by recipient age (≥70 years old versus <70 years old) and procedure type (single versus double lung transplant). Demographics and clinical variables were compared using Chi-square test and two sample t-test. Survival was assessed by Kaplan-Meier curves and compared by log-rank test with propensity score matching. RESULTS: 988 LTs were performed at our center over 10 years, including 289 LTs in patients ≥70 years old and 699 LTs in patients <70 years old. The recipient groups differed significantly by race (p < 0.0001), sex (p = 0.003), and disease etiology (p < 0.0001). Older patients were less likely to receive a double lung transplant compared to younger patients (p < 0.0001) and had lower rates of intraoperative cardiopulmonary bypass (p = 0.019) and shorter length of stay (p = 0.001). Both groups had overall high 1-year survival (85.8% versus 89.1%, respectively). Survival did not differ between groups after propensity matching (p = 0.15). CONCLUSIONS: Our data showed high survival for older and younger LT recipients. There were no statistically significant differences observed in survival between the groups after propensity matching, however, a trend in favour of younger patients was observed.

Rebuilding and rebooting immunity with stem cells.

Advances in modern medicine have enabled a rapid increase in lifespan and, consequently, have highlighted the immune system as a key driver of age-related disease. Immune regeneration therapies present exciting strategies to address age-related diseases by rebooting the host's primary lymphoid tissues or rebuilding the immune system directly via biomaterials or artificial tissue. Here, we identify important, unanswered questions regarding the safety and feasibility of these therapies. Further, we identify key design parameters that should be primary considerations guiding technology design, including timing of application, interaction with the host immune system, and functional characterization of the target patient population.