Study cohort
We enrolled four cases of intestinal perforation in neonates. Two cases were in patients with NEC (identified as NEC1 and NEC2), and the other two were intestinal perforation without necrosis. One of these was a patient with FIP (patient FIP1), and the other was a patient with idiopathic perforation (IDP, IDP1) due to a fecal plug. In all four cases, the perforated and necrotic intestines were surgically resected.
The details of the patients’ clinical history and postoperative course are shown in Table 1. All cases were operated on after abdominal X-rays revealed pneumoperitoneum that suggested intestinal perforation. The time from symptom onset to surgery ranged from a few hours to two days. For NEC1, blood tests conducted a day before surgery revealed abnormalities such as low platelets and low neutrophils. FIP1 exhibited abdominal distension two days before the surgery. For IDP1, enterography for a fecal plug a day before surgery revealed pneumoperitoneum. Puncture drainage was performed initially, but the condition did not improve and laparotomy was performed the next day. NEC2 exhibited sudden abdominal distension and was operated on a few hours later. NEC1 died from hepatic hemorrhage three days postoperatively. NEC2, FIP1, and IDP1 survived the postoperative course.
Table 1
Case | Organ | Age at surgery (days) | Gestational age (weeks) | Birth weight (g) | Outcome | Preoperative course |
NEC1 | Ileum | 6 | 24 | 699 | Died | Low platelets and low neutrophils 1 d before |
NEC2 | Ileum | 2 | 33 | 1,690 | Survived | Sudden abdominal distension on operative day |
FIP1 | Jejunum | 4 | 27 | 707 | Survived | Abdominal distension 2 d before |
IDP1 | Ileum | 4 | 28 | 737 | Survived | Pneumoperitoneum and puncture drained 1 d before |
NEC, necrotizing enterocolitis; FIP, focal intestinal perforation; IDP, idiopathic perforation |
scRNA-seq library preparation and sequencing
We removed intestinal epithelium from the resected intestine and extracted live mononuclear cells to perform scRNA-seq of the immune cells in the intestinal lamina propria. The scRNA-seq library preparation was performed using the 10× Chromium system. For each case, we sequenced an average of 400 million reads for around 10,000 cells (NEC1: 8,312 cells; NEC2: 9,001 cells; FIP1: 8,295 cells; IDP1: 13,882 cells).
Cell type identification
Using the statistical analysis software R and packages for scRNA-seq (Seurat[22] and singleR[24]), we identified cell types from the gene expression data (Fig. 1A). Since most neutrophils were removed during mononuclear cell isolation, minor neutrophilic contamination was excluded from the analysis. We identified major immune cells such as T cells (15.1–47.7%), B cells (3.1–19.0%), monocytes (16.5–31.2%), macrophages (1.6–17.4%), DCs (2.4–12.2%), and NK cells (7.5–12.8%) among the blood mononuclear cells (BMCs) from each patient (Fig. 1B). They were distributed in similar proportions to those in the neonatal cord blood[27], based on the fact that T cells and monocytes comprised the majority of cells in the non-neutrophil category. The proportion of cell types differed among the patients, however. NEC2 had more T cells and B cells than the other patients, while IDP1 had fewer T cells and more macrophages and DCs. We did not identify any rare immune cells such as ILCs, either using singleR or based on the expression of transcription factors such as GATA3 and RORC. The identification of cell types was consistent with the expression of several marker genes.
We found non-BMCs such as smooth muscle cells, fibroblastic cells, endothelial cells, and other mesenchymal cells that remained after BMC isolation (Fig. 1C). However, we did not analyze these cells because they were essentially contaminants that remained after the isolation of BMCs.
Gene set enrichment analysis
We compared the gene-expression data for T cells, B cells, monocytes, and macrophages between the NEC (NEC1 and NEC2) and non-NEC (FIP1 and IDP1) groups using gene set enrichment analysis (GSEA; Fig. 2). The pathways that were enriched in the NEC group were mainly related to upregulated inflammatory reactions (MTOR signaling: HALLMARK_MTORC1_SIGNALING; TNF-α signaling: HALLMARK_TNFA_SIGNALING_VIA_NFKB) and cell proliferation (MYC signaling: HALLMARK_MYC_TARGETS_V1). Genes associated with the MTOR signaling pathway were upregulated in the NEC group in T cells, B cells, monocytes, and macrophages (normalized enrichment score [NES] = 1.84, 1.91, 1.86, 1.60, respectively; false discovery rate [FDR] = 0.031, 0.031, 0.030, 0.084, respectively).
Subsets of cell types
Within the cell types identified by singleR, we tried to identify cell subsets based on marker-gene expression.
Among T cells, we identified naïve, effector, memory, and Treg cells[28–30] (Fig. 3A, B). We were not able to completely discriminate between CD4+ and CD8+ naïve T cells because the expression of lineage markers in these cells was either low or negative, based on our scRNA-seq analysis. All four cases exhibited a high proportion of Th1 cells, but we did not identify any T helper 2 (Th2) cells (Fig. 3C). The cell counts of Th1 cells in the NEC group (11.0% and 15.1%) were lower than in the non-NEC group (36.1% and 44.3%). In addition to Th1 cells, we identified T helper 17 (Th17) (0.3–9.0%) and CD8+ effector T (Teff) cells (0.15–19.0%), which are usually present in Th1-biased responses[31]. Treg cells were also present (2.6–13.8%). The ratio of Th17 to Treg cell counts[32, 33] varied from 0.11 to 1.40. The proportion of CD8+ central memory T (TCM) cells for NEC1 was > 100 times greater than that for NEC2 (41.1% vs 0.36%) and higher than for the non-NEC cases (8.6% and 10.6% in FIP1 and IDP1, respectively), although the reason for this was unclear. The proportions of cell subsets for NEC2 differed substantially from those for the other three patients: the frequency of naïve T cells was higher (81.8%), and the frequencies of CD4+ TCM cells (0%), CD8+ TCM cells (0.36%), and CD8+ Teff cells (0.15%) were lower.
Among the B cells, we identified activated B cells, memory B cells, and long-lived plasma cells[34, 35] (Fig. 4A, B). However, only one of these subsets of B cells was predominant in each patient. There were more activated B cells in NEC2 (93.7%), memory B cells in NEC1 (96.3%) and FIP1 (85.6%), and long-lived plasma cells in IDP1 (93.0%) (Fig. 4C). These differences may be explained by the patient’s stage of inflammation: NEC2 may have been in the earliest phase of inflammation, NEC1 and FIP1 may have been in the middle phases, and IDP1 may have been in the final phase.
We differentiated between classical monocytes and nonclassical monocytes (Fig. 4D, E, F), although we could not identify two clusters using known monocyte markers (e.g., CD14, CD16, CD55, CD115, CD127, and CCR2)[36, 37]. Among the macrophages, we did not identify well-known subsets such as M1 and M2 macrophages, even using typical markers[38–40], possibly because of alterations in gene expression caused by inflammation (Supplemental Fig. 2A, B). We were also unable to clearly identify subsets of DCs and NK cells, possibly because the classification of subsets and markers for these groups is still controversial[41–44] (Supplemental Fig. 2C, D, E, F).