testDS_LMM: Test for differential states: method 'diffcyt-DS-LMM'

In diffcyt: Differential discovery in high-dimensional cytometry via high-resolution clustering

Description

Calculate tests for differential states within cell populations using method 'diffcyt-DS-LMM'

Usage

testDS_LMM(
  d_counts,
  d_medians,
  formula,
  contrast,
  weights = TRUE,
  markers_to_test = NULL,
  min_cells = 3,
  min_samples = NULL
)

Arguments

d_counts	SummarizedExperiment object containing cluster cell counts, from calcCounts.
d_medians	SummarizedExperiment object containing cluster medians (median marker expression for each cluster-sample combination), from calcMedians. Assumed to contain a logical vector id_state_markers in the meta-data (accessed with metadata(d_medians)$id_state_markers), which identifies the set of 'cell state' markers in the list of assays.
formula	Model formula object, created with createFormula. This should be a list containing three elements: formula, data, and random_terms: the model formula, data frame of corresponding variables, and variable indicating whether the model formula contains any random effect terms. See createFormula for details.
contrast	Contrast matrix, created with createContrast. See createContrast for details.
weights	(Optional) Whether to include precision weights within each model (across samples, i.e. within the model for each cluster); these represent the relative uncertainty in calculating each median value (within each model). Accepts values of TRUE, FALSE, or a numeric vector of custom weights. Default = TRUE, in which case cluster cell counts are used as weights.
markers_to_test	(Optional) Logical vector specifying which markers to test for differential expression (from the set of markers stored in the assays of d_medians). Default = all 'cell state' markers, which are identified by the logical vector id_state_markers stored in the meta-data of d_medians.
min_cells	Filtering parameter. Default = 3. Clusters are kept for differential testing if they have at least min_cells cells in at least min_samples samples.
min_samples	Filtering parameter. Default = number of samples / 2, which is appropriate for two-group comparisons (of equal size). Clusters are kept for differential testing if they have at least min_cells cells in at least min_samples samples.

Details

Calculates tests for differential states within cell populations (i.e. differential expression of cell state markers within clusters), using linear mixed models (LMMs). Clusters are defined using cell type markers, and cell states are characterized by the median transformed expression of cell state markers.

This methodology was originally developed and described by Nowicka et al. (2017), F1000Research, and has been modified here to make use of high-resolution clustering to enable investigation of rare cell populations. Note that unlike the original method by Nowicka et al., we do not attempt to manually merge clusters into canonical cell populations. Instead, results are reported at the high-resolution cluster level, and the interpretation of significant differential clusters is left to the user via visualizations such as heatmaps (see the package vignette for an example).

This method fits linear mixed models (LMMs) for each cluster-marker combination (cell state markers only), and calculates differential tests separately for each cluster-marker combination. The response variable in each model is the median arcsinh-transformed marker expression of the cell state marker, which is assumed to follow a Gaussian distribution. There is one model per cluster per cell state marker. Within each model, sample-level weights are included (by default) for the number of cells per sample; these weights represent the relative uncertainty in calculating each median value. (Additional uncertainty exists due to variation in the total number of cells per cluster; however, it is not possible to account for this, since there are separate models for each cluster-marker combination.) We also include a filtering step to remove clusters with very small numbers of cells, to improve statistical power.

For more details on the statistical methodology, see Nowicka et al. (2017), F1000Research (section 'Differential analysis of marker expression stratified by cell population'.)

The experimental design must be specified using a model formula, which can be created with createFormula. Flexible experimental designs are possible, including blocking (e.g. paired designs), batch effects, and continuous covariates. Blocking variables can be included as either random intercept terms or fixed effect terms (see createFormula). For paired designs, we recommend using random intercept terms to improve statistical power; see Nowicka et al. (2017), F1000Research for details. Batch effects and continuous covariates should be included as fixed effects.

If no random intercept terms are included in the model formula, model fitting is performed using a linear model (LM) instead of a LMM.

The contrast matrix specifying the contrast of interest can be created with createContrast. See createContrast for more details.

By default, differential tests are performed for all cell state markers (which are identified with the vector id_state_markers stored in the meta-data of the cluster medians input object). The optional argument markers_to_test allows the user to specify a different set of markers to test (e.g. to investigate differences for cell type markers).

Filtering: Clusters are kept for differential testing if they have at least min_cells cells in at least min_samples samples. This removes clusters with very low cell counts across conditions, to improve power.

Weights: By default, cluster cell counts are used as precision weights within each model (across samples only, i.e. within the model for each cluster); these represent the relative uncertainty in calculating each median value (within each model). See above for details.

Value

Returns a new SummarizedExperiment object, where rows = cluster-marker combinations, and columns = samples. In the rows, clusters are repeated for each cell state marker (i.e. the sheets or assays from the previous d_medians object are stacked into a single matrix). Differential test results are stored in the rowData slot. Results include raw p-values (p_val) and adjusted p-values (p_adj), which can be used to rank cluster-marker combinations by evidence for differential states within cell populations. The results can be accessed with the rowData accessor function.

Examples

# For a complete workflow example demonstrating each step in the 'diffcyt' pipeline, 
# see the package vignette.

# Function to create random data (one sample)
d_random <- function(n = 20000, mean = 0, sd = 1, ncol = 20, cofactor = 5) {
  d <- sinh(matrix(rnorm(n, mean, sd), ncol = ncol)) * cofactor
  colnames(d) <- paste0("marker", sprintf("%02d", 1:ncol))
  d
}

# Create random data (without differential signal)
set.seed(123)
d_input <- list(
  sample1 = d_random(), 
  sample2 = d_random(), 
  sample3 = d_random(), 
  sample4 = d_random()
)

# Add differential states (DS) signal
ix_DS <- 901:1000
ix_cols_type <- 1:10
ix_cols_DS <- 19:20
d_input[[1]][ix_DS, ix_cols_type] <- d_random(n = 1000, mean = 3, ncol = 10)
d_input[[2]][ix_DS, ix_cols_type] <- d_random(n = 1000, mean = 3, ncol = 10)
d_input[[3]][ix_DS, c(ix_cols_type, ix_cols_DS)] <- d_random(n = 1200, mean = 3, ncol = 12)
d_input[[4]][ix_DS, c(ix_cols_type, ix_cols_DS)] <- d_random(n = 1200, mean = 3, ncol = 12)

experiment_info <- data.frame(
  sample_id = factor(paste0("sample", 1:4)), 
  group_id = factor(c("group1", "group1", "group2", "group2")), 
  stringsAsFactors = FALSE
)

marker_info <- data.frame(
  channel_name = paste0("channel", sprintf("%03d", 1:20)), 
  marker_name = paste0("marker", sprintf("%02d", 1:20)), 
  marker_class = factor(c(rep("type", 10), rep("state", 10)), 
                        levels = c("type", "state", "none")), 
  stringsAsFactors = FALSE
)

# Prepare data
d_se <- prepareData(d_input, experiment_info, marker_info)

# Transform data
d_se <- transformData(d_se)

# Generate clusters
d_se <- generateClusters(d_se)

# Calculate counts
d_counts <- calcCounts(d_se)

# Calculate medians
d_medians <- calcMedians(d_se)

# Create model formula
formula <- createFormula(experiment_info, cols_fixed = "group_id")

# Create contrast matrix
contrast <- createContrast(c(0, 1))

# Test for differential states (DS) within clusters
res_DS <- testDS_LMM(d_counts, d_medians, formula, contrast)

diffcyt documentation built on Nov. 8, 2020, 6:37 p.m.