Demultiplexing with hashtag oligos (HTOs)

Compiled: 2021-06-14

Source: vignettes/hashing_vignette.Rmd

Developed in collaboration with the Technology Innovation Group at NYGC, Cell Hashing uses oligo-tagged antibodies against ubiquitously expressed surface proteins to place a “sample barcode” on each single cell, enabling different samples to be multiplexed together and run in a single experiment. For more information, please refer to this paper.

This vignette will give a brief demonstration on how to work with data produced with Cell Hashing in Seurat. Applied to two datasets, we can successfully demultiplex cells to their the original sample-of-origin, and identify cross-sample doublets.

The demultiplexing function HTODemux() implements the following procedure:
  • We perform a k-medoid clustering on the normalized HTO values, which initially separates cells into K(# of samples)+1 clusters.
  • We calculate a ‘negative’ distribution for HTO. For each HTO, we use the cluster with the lowest average value as the negative group.
  • For each HTO, we fit a negative binomial distribution to the negative cluster. We use the 0.99 quantile of this distribution as a threshold.
  • Based on these thresholds, each cell is classified as positive or negative for each HTO.
  • Cells that are positive for more than one HTOs are annotated as doublets.

8-HTO dataset from human PBMCs

Dataset description:
  • Data represent peripheral blood mononuclear cells (PBMCs) from eight different donors.
  • Cells from each donor are uniquely labeled, using CD45 as a hashing antibody.
  • Samples were subsequently pooled, and run on a single lane of the the 10X Chromium v2 system.
  • You can download the count matrices for RNA and HTO here, or the FASTQ files from GEO

Basic setup

Load packages

Read in data

# Load in the UMI matrix
pbmc.umis <- readRDS("../data/pbmc_umi_mtx.rds")

# For generating a hashtag count matrix from FASTQ files, please refer to
# https://github.com/Hoohm/CITE-seq-Count.  Load in the HTO count matrix
pbmc.htos <- readRDS("../data/pbmc_hto_mtx.rds")

# Select cell barcodes detected by both RNA and HTO In the example datasets we have already
# filtered the cells for you, but perform this step for clarity.
joint.bcs <- intersect(colnames(pbmc.umis), colnames(pbmc.htos))

# Subset RNA and HTO counts by joint cell barcodes
pbmc.umis <- pbmc.umis[, joint.bcs]
pbmc.htos <- as.matrix(pbmc.htos[, joint.bcs])

# Confirm that the HTO have the correct names
rownames(pbmc.htos)
## [1] "HTO_A" "HTO_B" "HTO_C" "HTO_D" "HTO_E" "HTO_F" "HTO_G" "HTO_H"

Setup Seurat object and add in the HTO data

# Setup Seurat object
pbmc.hashtag <- CreateSeuratObject(counts = pbmc.umis)

# Normalize RNA data with log normalization
pbmc.hashtag <- NormalizeData(pbmc.hashtag)
# Find and scale variable features
pbmc.hashtag <- FindVariableFeatures(pbmc.hashtag, selection.method = "mean.var.plot")
pbmc.hashtag <- ScaleData(pbmc.hashtag, features = VariableFeatures(pbmc.hashtag))

Adding HTO data as an independent assay

You can read more about working with multi-modal data here

# Add HTO data as a new assay independent from RNA
pbmc.hashtag[["HTO"]] <- CreateAssayObject(counts = pbmc.htos)
# Normalize HTO data, here we use centered log-ratio (CLR) transformation
pbmc.hashtag <- NormalizeData(pbmc.hashtag, assay = "HTO", normalization.method = "CLR")

Demultiplex cells based on HTO enrichment

Here we use the Seurat function HTODemux() to assign single cells back to their sample origins.

# If you have a very large dataset we suggest using k_function = 'clara'. This is a k-medoid
# clustering function for large applications You can also play with additional parameters (see
# documentation for HTODemux()) to adjust the threshold for classification Here we are using
# the default settings
pbmc.hashtag <- HTODemux(pbmc.hashtag, assay = "HTO", positive.quantile = 0.99)

Visualize demultiplexing results

Output from running HTODemux() is saved in the object metadata. We can visualize how many cells are classified as singlets, doublets and negative/ambiguous cells.

# Global classification results
table(pbmc.hashtag$HTO_classification.global)
## 
##  Doublet Negative  Singlet 
##     2598      346    13972

Visualize enrichment for selected HTOs with ridge plots

# Group cells based on the max HTO signal
Idents(pbmc.hashtag) <- "HTO_maxID"
RidgePlot(pbmc.hashtag, assay = "HTO", features = rownames(pbmc.hashtag[["HTO"]])[1:2], ncol = 2)

Visualize pairs of HTO signals to confirm mutual exclusivity in singlets

FeatureScatter(pbmc.hashtag, feature1 = "hto_HTO-A", feature2 = "hto_HTO-B")

Compare number of UMIs for singlets, doublets and negative cells

Idents(pbmc.hashtag) <- "HTO_classification.global"
VlnPlot(pbmc.hashtag, features = "nCount_RNA", pt.size = 0.1, log = TRUE)

Generate a two dimensional tSNE embedding for HTOs.Here we are grouping cells by singlets and doublets for simplicity.

# First, we will remove negative cells from the object
pbmc.hashtag.subset <- subset(pbmc.hashtag, idents = "Negative", invert = TRUE)

# Calculate a tSNE embedding of the HTO data
DefaultAssay(pbmc.hashtag.subset) <- "HTO"
pbmc.hashtag.subset <- ScaleData(pbmc.hashtag.subset, features = rownames(pbmc.hashtag.subset),
    verbose = FALSE)
pbmc.hashtag.subset <- RunPCA(pbmc.hashtag.subset, features = rownames(pbmc.hashtag.subset), approx = FALSE)
pbmc.hashtag.subset <- RunTSNE(pbmc.hashtag.subset, dims = 1:8, perplexity = 100)
DimPlot(pbmc.hashtag.subset)

# You can also visualize the more detailed classification result by running Idents(object) <-
# 'HTO_classification' before plotting. Here, you can see that each of the small clouds on the
# tSNE plot corresponds to one of the 28 possible doublet combinations.

Create an HTO heatmap, based on Figure 1C in the Cell Hashing paper.

# To increase the efficiency of plotting, you can subsample cells using the num.cells argument
HTOHeatmap(pbmc.hashtag, assay = "HTO", ncells = 5000)

Cluster and visualize cells using the usual scRNA-seq workflow, and examine for the potential presence of batch effects.

# Extract the singlets
pbmc.singlet <- subset(pbmc.hashtag, idents = "Singlet")

# Select the top 1000 most variable features
pbmc.singlet <- FindVariableFeatures(pbmc.singlet, selection.method = "mean.var.plot")

# Scaling RNA data, we only scale the variable features here for efficiency
pbmc.singlet <- ScaleData(pbmc.singlet, features = VariableFeatures(pbmc.singlet))

# Run PCA
pbmc.singlet <- RunPCA(pbmc.singlet, features = VariableFeatures(pbmc.singlet))
# We select the top 10 PCs for clustering and tSNE based on PCElbowPlot
pbmc.singlet <- FindNeighbors(pbmc.singlet, reduction = "pca", dims = 1:10)
pbmc.singlet <- FindClusters(pbmc.singlet, resolution = 0.6, verbose = FALSE)
pbmc.singlet <- RunTSNE(pbmc.singlet, reduction = "pca", dims = 1:10)

# Projecting singlet identities on TSNE visualization
DimPlot(pbmc.singlet, group.by = "HTO_classification")

12-HTO dataset from four human cell lines

Dataset description:
  • Data represent single cells collected from four cell lines: HEK, K562, KG1 and THP1
  • Each cell line was further split into three samples (12 samples in total).
  • Each sample was labeled with a hashing antibody mixture (CD29 and CD45), pooled, and run on a single lane of 10X.
  • Based on this design, we should be able to detect doublets both across and within cell types
  • You can download the count matrices for RNA and HTO here, and are available on GEO here

Create Seurat object, add HTO data and perform normalization

# Read in UMI count matrix for RNA
hto12.umis <- readRDS("../data/hto12_umi_mtx.rds")

# Read in HTO count matrix
hto12.htos <- readRDS("../data/hto12_hto_mtx.rds")

# Select cell barcodes detected in both RNA and HTO
cells.use <- intersect(rownames(hto12.htos), colnames(hto12.umis))

# Create Seurat object and add HTO data
hto12 <- CreateSeuratObject(counts = hto12.umis[, cells.use], min.features = 300)
hto12[["HTO"]] <- CreateAssayObject(counts = t(x = hto12.htos[colnames(hto12), 1:12]))

# Normalize data
hto12 <- NormalizeData(hto12)
hto12 <- NormalizeData(hto12, assay = "HTO", normalization.method = "CLR")

Demultiplex data

hto12 <- HTODemux(hto12, assay = "HTO", positive.quantile = 0.99)

Visualize demultiplexing results

Distribution of selected HTOs grouped by classification, displayed by ridge plots

RidgePlot(hto12, assay = "HTO", features = c("HEK-A", "K562-B", "KG1-A", "THP1-C"), ncol = 2)

Visualize HTO signals in a heatmap

HTOHeatmap(hto12, assay = "HTO")

Visualize RNA clustering

  • Below, we cluster the cells using our standard scRNA-seq workflow. As expected we see four major clusters, corresponding to the cell lines
  • In addition, we see small clusters in between, representing mixed transcriptomes that are correctly annotated as doublets.
  • We also see within-cell type doublets, that are (perhaps unsurprisingly) intermixed with singlets of the same cell type
    • # Remove the negative cells
hto12 <- subset(hto12, idents = "Negative", invert = TRUE)

# Run PCA on most variable features
hto12 <- FindVariableFeatures(hto12, selection.method = "mean.var.plot")
hto12 <- ScaleData(hto12, features = VariableFeatures(hto12))
hto12 <- RunPCA(hto12)
hto12 <- RunTSNE(hto12, dims = 1:5, perplexity = 100)
DimPlot(hto12) + NoLegend()

    Session Info

    ## R version 4.1.0 (2021-05-18)
## Platform: x86_64-pc-linux-gnu (64-bit)
## Running under: Ubuntu 20.04.2 LTS
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##  [9] LC_ADDRESS=C               LC_TELEPHONE=C            
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## 
## attached base packages:
## [1] stats     graphics  grDevices utils     datasets  methods   base     
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## other attached packages:
## [1] SeuratObject_4.0.2 Seurat_4.0.3      
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