1 Data examples

2 Normalization

CATALYST provides an implementation of bead-based normalization as described by Finck et al. (2). Here, identification of bead-singlets (used for normalization), as well as of bead-bead and cell-bead doublets (to be removed) is automated as follows:

  1. beads are identified as events with their top signals in the bead channels
  2. cell-bead doublets are remove by applying a separation cutoff to the distance between the lowest bead and highest non-bead signal
  3. events passing all vertical gates defined by the lower bounds of bead signals are removed (these include bead-bead and bead-cell doublets)
  4. bead-bead doublets are removed by applying a default \(median\;\pm5\;mad\) rule to events identified in step 2. The remaining bead events are used for normalization.

2.1 Normalization work-flow

2.1.1 concatFCS: Concatination of FCS files

Multiple flowFrames or FCS files can be concatenated via concatFCS, which takes either a flowSet, a list of flowFrames, a character specifying the location of the FCS files to be concatinated, or a vector of FCS file names as input. If out_path=NULL (the default), the function will return a single flowFrame containing the measurement data of all files. Otherwise, an FCS 3.0 standard file of the concatenated data will be written to the specified location.

ff <- concatFCS(raw_data)

2.1.2 normCytof: Normalization using bead standards

Since bead gating is automated here, normalization comes down to a single function that takes a flowFrame as input and only requires specification of the beads to be used for normalization. Valid options are:

  • "dvs" for bead masses 140, 151, 153, 165, 175
  • "beta" for bead masses 139, 141, 159, 169, 175
  • or a custom numeric vector of bead masses

By default, we apply a \(median\;\pm5\;mad\) rule to remove low- and high-signal events from the bead population used for estimating normalization factors. The extent to which bead populations are trimmed can be adjusted via trim. The population will become increasingly narrow and bead-bead doublets will be exluded as the trim value decreases. Notably, slight over-trimming will not affect normalization. It is therefore recommended to choose a trim value that is small enough to assure removal of doublets at the cost of a small bead population to normalize to.

normCytof(x=ff, y="dvs", k=80, plot=FALSE)

3 Debarcoding

CATALYST provides an implementation of the single-cell deconvolution algorithm described by Zunder et al. (3). The package contains three functions for debarcoding and three visualizations that guide selection of thresholds and give a sense of barcode assignment quality.

In summary, events are assigned to a sample when i) their positive and negative barcode populations are separated by a distance larger than a threshold value and ii) the combination of their positive barcode channels appears in the barcoding scheme. Depending on the supplied scheme, there are two possible ways of arriving at preliminary event assignments:

  1. Doublet-filtering:
    Given a binary barcoding scheme with a coherent number \(k\) of positive channels for all IDs, the \(k\) highest channels are considered positive and \(n-k\) channels negative. Separation of positive and negative events equates to the difference between the \(k\)th highest and \((n-k)\)th lowest intensity value. If a numeric vector of masses is supplied, the barcoding scheme will be an identity matrix; the most intense channel is considered positive and its respective mass assigned as ID.
  2. Non-constant number of 1’s:
    Given a non-uniform number of 1’s in the binary codes, the highest separation between consecutive barcodes is looked at. In both, the doublet-filtering and the latter case, each event is assigned a binary code that, if matched with a code in the barcoding scheme supplied, dictates which row name will be assigned as ID. Cells whose positive barcodes are still very low or whose binary pattern of positive and negative barcodes doesn’t occur in the barcoding scheme will be given ID 0 for “unassigned”.

All data required for debarcoding are held in objects of class dbFrame (see Appendix), allowing for the following easy-to-use work-flow:

  1. as the initial step of single-cell deconcolution, assignPrelim will return a dbFrame containing the input measurement data, barcoding scheme, and preliminary event assignments.
  2. assignments will be made final by applyCutoffs. It is recommended to estimate, and possibly adjust, population-specific separation cutoffs by running estCutoffs prior to this.
  3. plotYields, plotEvents and plotMahal aim to guide selection of devoncolution parameters and to give a sense of the resulting barcode assignment quality.
  4. lastly, population-wise FCS files are written from the dbFrame with outFCS.

3.1 Debarcoding work-flow

3.1.1 assignPrelim: Assignment of preliminary IDs

The debarcoding process commences by assigning each event a preliminary barcode ID. assignPrelim thereby takes either a binary barcoding scheme or a vector of numeric masses as input, and accordingly assigns each event the appropirate row name or mass as ID. FCS files are read into R with read.FCS of the flowCore package, and are represented as an object of class flowFrame:

## flowFrame object 'anonymous'
## with 20000 cells and 6 observables:
##        name  desc    range   minRange maxRange
## 1 (Pd102)Di BC102 9745.799 -0.9999121 9745.799
## 2 (Pd104)Di BC104 9687.522 -0.9994696 9687.522
## 3 (Pd105)Di BC105 8924.638 -0.9989271 8924.638
## 4 (Pd106)Di BC106 8016.669 -0.9997822 8016.669
## 5 (Pd108)Di BC108 9043.869 -0.9999974 9043.869
## 6 (Pd110)Di BC110 8204.455 -0.9999368 8204.455
## 0 keywords are stored in the 'description' slot

The debarcoding scheme should be a binary table with sample IDs as row and numeric barcode masses as column names:

##    102 104 105 106 108 110
## A1   1   1   1   0   0   0
## A2   1   1   0   1   0   0
## A3   1   1   0   0   1   0
## A4   1   1   0   0   0   1
## A5   1   0   1   1   0   0
## B1   1   0   1   0   1   0

Provided with a flowFrame and a compatible barcoding scheme (barcode masses must occur in the parameters of the flowFrame), assignPrelim will return a dbFrame containing exprs from the input flowFrame, a numeric or character vector of event assignments in slot bc_ids, separations between barcode populations on the normalized scale in slot deltas, normalized barcode intensities in slot normed_bcs, and the counts and yields matrices. Measurement intensities are normalized by population such that each is scaled to the 95% quantile of asinh transformed measurement intensities of events assigned to the respective barcode population.

re0 <- assignPrelim(x=sample_ff, y=sample_key, verbose=FALSE)
## dbFrame object with
## 20000 events, 6 observables and 20 barcodes:
## Current assignments:
##       0 event(s) unassigned                                                                           
## ID    A1   A2   A3   A4   A5   B1   B2   B3   B4   B5   C1   C2   C3   C4  
## Count 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
## ID    C5   D1   D2   D3   D4   D5  
## Count 1000 1000 1000 1000 1000 1000

3.1.2 estCutoffs: Estimation of separation cutoffs

As opposed to a single global cutoff, estCutoffs will estimate a sample-specific cutoff to deal with barcode population cell yields that decline in an asynchronous fashion. Thus, the choice of thresholds for the distance between negative and positive barcode populations can be i) automated and ii) independent for each barcode. Nevertheless, reviewing the yield plots (see below), checking and possibly refining separation cutoffs is advisable.

For the estimation of cutoff parameters we consider yields upon debarcoding as a function of the applied cutoffs. Commonly, this function will be characterized by an initial weak decline, where doublets are excluded, and subsequent rapid decline in yields to zero. Inbetween, low numbers of counts with intermediate barcode separation give rise to a plateau. To facilitate robust estimation, we fit a linear and a three-parameter log-logistic function (4) to the yields function with the LL.3 function of the drc R package (5) (Figure 1). As an adequate cutoff estimate, we target a point that marks the end of the plateau regime and on-set of yield decline to appropriately balance confidence in barcode assignment and cell yield.

The goodness of the linear fit relative to the log-logistic fit is weighed as follow: \[w = \frac{\text{RSS}_{log-logistic}}{\text{RSS}_{log-logistic}+\text{RSS}_{linear}}\]

The cutoffs for both functions are defined as:

\[c_{linear} = -\frac{\beta_0}{2\beta_1}\] \[c_{log-logistic}=\underset{x}{\arg\min}\:\frac{\vert\:f'(x)\:\vert}{f(x)} > 0.1\]

The final cutoff estimate \(c\) is defined as the weighted mean between these estimates:

\[c=(1-w)\cdot c_{log-logistic}+w\cdot c_{linear}\]