celltypist 0.2.1

A tool for semi-automatic cell type annotation

CellTypist is an automated cell type annotation tool for scRNA-seq datasets on the basis of logistic regression classifiers optimised by the stochastic gradient descent algorithm. CellTypist allows for cell prediction using either built-in (with a current focus on immune sub-populations) or custom models, in order to assist in the accurate classification of different cell types and subtypes.

CellTypist website

Information of CellTypist can be also found in our CellTypist portal.

Interactive tutorials

Using CellTypist for cell type classification
Using CellTypist for multi-label classification
Best practice in large-scale cross-dataset label transfer using CellTypist

Install CellTypist

Using pip

pip install celltypist

Using conda

conda install -c bioconda -c conda-forge celltypist

Usage

1. Use in the Python environment

• 1.1. Import the module

  import celltypist
  from celltypist import models
  

• 1.2. Download available models

  The models serve as the basis for cell type predictions. Information of available models can be also found here.

  #Show all available models that can be downloaded and used.
  models.models_description()
  #Download a specific model, for example, `Immune_All_Low.pkl`.
  models.download_models(model = 'Immune_All_Low.pkl')
  #Download a list of models, for example, `Immune_All_Low.pkl` and `Immune_All_High.pkl`.
  models.download_models(model = ['Immune_All_Low.pkl', 'Immune_All_High.pkl'])
  #Update the models by re-downloading the latest versions if you think they may be outdated.
  models.download_models(model = ['Immune_All_Low.pkl', 'Immune_All_High.pkl'], force_update = True)
  #Show the local directory storing these models.
  models.models_path
  

  A simple way is to download all available models. Since each model is on average 1 megabyte (MB), we encourage the users to download all of them.

  #Download all the available models.
  models.download_models()
  #Update all models by re-downloading the latest versions if you think they may be outdated.
  models.download_models(force_update = True)
  

  By default, a folder .celltypist/ will be created in the user's home directory to store model files. A different path/folder can be specified by exporting the environment variable CELLTYPIST_FOLDER in your configuration file (e.g. in ~/.bash_profile).

  #In the shell configuration file.
  export CELLTYPIST_FOLDER='/path/to/model/folder/'
  

• 1.3. Overview of the models

  All models are serialised in a binary format by pickle.

  #Get an overview of the models that are downloaded in `1.2.`.
  #By default (`on_the_fly = False`), all possible models (even those that are not downloaded) are shown.
  models.models_description(on_the_fly = True)
  

• 1.4. Inspect the model of interest

  To take a look at a given model, load the model as an instance of the Model class as defined in CellTypist.

  #Select the model from the above list. If the `model` argument is not provided, will default to `Immune_All_Low.pkl`.
  model = models.Model.load(model = 'Immune_All_Low.pkl')
  #The model summary information.
  model
  #Examine cell types contained in the model.
  model.cell_types
  #Examine genes/features contained in the model.
  model.features
  

• 1.5. Celltyping based on the input of count table

  CellTypist accepts the input data as a count table (cell-by-gene or gene-by-cell) in the format of .txt, .csv, .tsv, .tab, .mtx or .mtx.gz. A raw count matrix (reads or UMIs) is required. Non-expressed genes (if you are sure of their expression absence in your data) are suggested to be included in the input table as well, as they point to the negative transcriptomic signatures when compared with the model used.

  #Get a demo test data. This is a UMI count csv file with cells as rows and gene symbols as columns.
  input_file = celltypist.samples.get_sample_csv()
  

  Assign the cell type labels from the model to the input test cells using the celltypist.annotate function.

  #Predict the identity of each input cell.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl')
  #Alternatively, the model argument can be a previously loaded `Model` as in 1.4.
  predictions = celltypist.annotate(input_file, model = model)
  

  If your input file is in a gene-by-cell format (genes as rows and cells as columns), pass in the transpose_input = True argument. In addition, if the input is provided in the .mtx format, you will also need to specify the gene_file and cell_file arguments as the files containing names of genes and cells, respectively.

  #In case your input file is a gene-by-cell table.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl', transpose_input = True)
  #In case your input file is a gene-by-cell mtx file.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl', transpose_input = True, gene_file = '/path/to/gene/file.txt', cell_file = '/path/to/cell/file.txt')
  

  Again, if the model argument is not specified, CellTypist will by default use the Immune_All_Low.pkl model.

  The annotate function will return an instance of the AnnotationResult class as defined in CellTypist.

  #Summary information for the prediction result.
  predictions
  #Examine the predicted cell type labels.
  predictions.predicted_labels
  #Examine the matrix representing the decision score of each cell belonging to a given cell type.
  predictions.decision_matrix
  #Examine the matrix representing the probability each cell belongs to a given cell type (transformed from decision matrix by the sigmoid function).
  predictions.probability_matrix
  

  By default, with the annotate function, each query cell is predicted into the cell type with the largest score/probability among all possible cell types (mode = 'best match'). This mode is straightforward and can be used to differentiate between highly homogeneous cell types.

  However, in some scenarios where a query cell cannot be assigned to any cell type in the reference model (i.e., a novel cell type) or can be assigned to multiple cell types (i.e., multi-label classification), a mode of probability match can be turned on (mode = 'prob match') with a probability cutoff (default to 0.5, p_thres = 0.5) to decide the cell types (none, 1, or multiple) assigned for a given cell.

  #Query cell will get the label of 'Unassigned' if it fails to pass the probability cutoff in each cell type.
  #Query cell will get multiple label outputs (concatenated by '|') if more than one cell type passes the probability cutoff.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl', mode = 'prob match', p_thres = 0.5)
  

  The three tables in the AnnotationResult (.predicted_labels, .decision_matrix and .probability_matrix) can be written out to local files (tables) by the function to_table, specifying the target folder for storage and the prefix common to each table.

  #Export the three results to csv tables.
  predictions.to_table(folder = '/path/to/a/folder', prefix = '')
  #Alternatively, export the three results to a single Excel table (.xlsx).
  predictions.to_table(folder = '/path/to/a/folder', prefix = '', xlsx = True)
  

  The resulting AnnotationResult can be also transformed to an AnnData which stores the expression matrix in the log1p normalised format (to 10,000 counts per cell) by the function to_adata. The predicted cell type labels can be inserted to this AnnData as well by specifying insert_labels = True (which is the default behavior of to_adata).

  Confidence scores of query cells can be inserted by specifying insert_conf = True (which is also the default behavior of to_adata). The scores correspond to the probabilities of cell predictions based on either predictions.predicted_labels.predicted_labels or predictions.predicted_labels.majority_voting (see 1.7.), which can be specified by insert_conf_by (default to the former, predicted_labels).

  #Get an `AnnData` with predicted labels and confidence scores embedded into the observation metadata columns.
  adata = predictions.to_adata(insert_labels = True, insert_conf = True)
  #Inspect these columns (`predicted_labels` and `conf_score`).
  adata.obs
  

  In addition, you can insert the decision matrix into the AnnData by passing in insert_decision = True, which represents the decision scores of each cell type distributed across the input cells. Alternatively, setting insert_prob = True will insert the probability matrix into the AnnData. The latter is the recommended way as probabilities are more interpretable (though sometimes not all query datasets converge to a meaningful range of probability values).

  After the insertion, multiple columns will show up in the cell metadata of AnnData, with each column's name as a cell type name. Of note, all these columns (including the predicted_labels and conf_score) can be prefixed with a specific string by setting prefix in to_adata.

  #Get an `AnnData` with predicted labels, confidence scores, and decision matrix.
  adata = predictions.to_adata(insert_labels = True, insert_conf = True, insert_decision = True)
  #Get an `AnnData` with predicted labels, confidence scores, and probability matrix (recommended).
  adata = predictions.to_adata(insert_labels = True, insert_conf = True, insert_prob = True)
  

  You can now manipulate this object with any functions or modules applicable to AnnData. Actually, CellTypist provides a quick function to_plots to visualise your AnnotationResult and store the figures without the need of explicitly transforming it into an AnnData.

  #Visualise the predicted cell types overlaid onto the UMAP.
  predictions.to_plots(folder = '/path/to/a/folder', prefix = '')
  

  A different prefix for the output figures can be specified with the prefix tag, and UMAP coordinates will be generated for the input dataset using a canonical Scanpy pipeline. The labels in the figure may be crowded if too many cell types are predicted (can be alleviated by a majority voting process, see 1.7.).

  If you also would like to inspect the decision score and probability distributions for each cell type involved in the model, pass in the plot_probability = True argument. This may take a bit longer time as one figure will be generated for each of the cell types from the model.

  #Visualise the decision scores and probabilities of each cell type overlaid onto the UMAP as well.
  predictions.to_plots(folder = '/path/to/a/folder', prefix = '', plot_probability = True)
  

  Multiple figures will be generated, including the predicted cell type labels overlaid onto the UMAP space, plus the decision score and probability distributions of each cell type on the UMAP.

• 1.6. Celltyping based on AnnData

  CellTypist also accepts the input data as an AnnData generated from for example Scanpy.

  Since the expression of each gene will be centred and scaled by matching with the mean and standard deviation of that gene in the provided model, CellTypist requires a logarithmised and normalised expression matrix stored in the AnnData (log1p normalised expression to 10,000 counts per cell). CellTypist will try the .X attribute first, and if it does not suffice, try the .raw.X attribute. If none of them fit into the desired data type or the expression matrix is not properly normalised, an error will be raised.

  #Provide the input as an `AnnData`.
  predictions = celltypist.annotate('/path/to/input.h5ad', model = 'Immune_All_Low.pkl')
  #Alternatively, the input can be specified as an `AnnData` already loaded in memory.
  predictions = celltypist.annotate(a_loaded_adata, model = 'Immune_All_Low.pkl')
  

  All the parameters and downstream operations are the same as in 1.5., except that 1) the transformed AnnData from to_adata stores all the expression matrix and other information as is in the original object. 2) when generating the visualisation figures, existing UMAP coordinates will be used. If no UMAP coordinates are found, CellTypist will fall back on the neighborhood graph to yield new 2D UMAP projections. If none is available, a canonical Scanpy pipeline will be performed to generate the UMAP coordinates as in 1.5..

  Of note, when the input is an AnnData, compared to the visualisations in 1.5., a more useful way for visualising the prediction result is to use the function celltypist.dotplot, which quantitatively compares the CellTypist prediction result with the cell types (or clusters) pre-defined and stashed in the AnnData by the user. Specifically, a dot plot will be generated, demonstrating the match between CellTypist predictions and manual annotations (or clusters). For each cell type or cluster (each column within the dot plot), this plot shows how it can be 'decomposed' into different cell types predicted by CellTypist.

  #Examine the correspondence between CellTypist predictions (`use_as_prediction`) and manual annotations (`use_as_reference`).
  #Here, `predicted_labels` from `predictions.predicted_labels` is used as the prediction result from CellTypist.
  #`use_as_prediction` can be also set as `majority_voting` (see `1.7.`).
  celltypist.dotplot(predictions, use_as_reference = 'column_key_of_manual_annotation', use_as_prediction = 'predicted_labels')
  

  Check celltypist.dotplot for other parameters controlling visualisation details of this plot.

• 1.7. Use a majority voting classifier combined with celltyping

  By default, CellTypist will only do the prediction jobs to infer the identities of input cells, which renders the prediction of each cell independent. To combine the cell type predictions with the cell-cell transcriptomic relationships, CellTypist offers a majority voting approach based on the idea that similar cell subtypes are more likely to form a (sub)cluster regardless of their individual prediction outcomes. To turn on the majority voting classifier in addition to the CellTypist predictions, pass in majority_voting = True to the annotate function.

  #Turn on the majority voting classifier as well.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl', majority_voting = True)
  

  During the majority voting, to define cell-cell relations, CellTypist will use a heuristic over-clustering approach according to the size of the input data with the aid of a Leiden clustering pipeline. Users can also provide their own over-clustering result to the over_clustering argument. This argument can be specified in several ways:

  1. an input plain file with the over-clustering result of one cell per line.
  2. a string key specifying an existing cell metadata column in the AnnData (pre-created by the user).
  3. a list-like object (such as a numpy 1D array) indicating the over-clustering result of all cells.
  4. if none of the above is provided, will use a heuristic over-clustering approach, noted above.

  #Add your own over-clustering result.
  predictions = celltypist.annotate(input_file, model = 'Immune_All_Low.pkl', majority_voting = True, over_clustering = '/path/to/over_clustering/file')
  

  There is also a min_prop parameter (defaults to 0) which controls the minimum proportion of cells from the dominant cell type required to name a given subcluster by this cell type. Subcluster that fails to pass this proportion threshold will be assigned Heterogeneous.

  Similarly, an instance of the AnnotationResult class will be returned.

  #Examine the predicted cell type labels.
  predictions.predicted_labels
  #Examine specifically the majority-voting results.
  predictions.predicted_labels.majority_voting
  #Examine the matrix representing the decision score of each cell belonging to a given cell type.
  predictions.decision_matrix
  #Examine the matrix representing the probability each cell belongs to a given cell type (transformed from decision matrix by the sigmoid function).
  predictions.probability_matrix
  

  Compared to the results without majority-voting functionality as in 1.5. and 1.6., the .predicted_labels attribute now has two extra columns (over_clustering and majority_voting) in addition to the column predicted_labels.

  Other parameters and downstream operations are the same as in 1.5. and 1.6.. Note that due to the majority-voting results added, the exported tables (by to_table), the transformed AnnData (by to_adata), and the visualisation figures (by to_plots) will all have additional outputs or information indicating the majority-voting outcomes. For example, when using the function celltypist.dotplot, you can set use_as_prediction = 'majority_voting' to visualise the match between majority-voting results with manual annotations. The other example is that when using to_adata, you can specify insert_conf_by = 'majority_voting' to have the confidence scores corresponding to the majority-voting result instead of raw predictions (insert_conf_by = 'predicted_labels' which is the default).

  #Examine the correspondence between CellTypist predictions (`use_as_prediction`) and manual annotations (`use_as_reference`).
  celltypist.dotplot(predictions, use_as_reference = 'column_key_of_manual_annotation', use_as_prediction = 'majority_voting')
  

2. Use as the command line

• 2.1. Check the command line options

  celltypist --help
  

• 2.2. Download all available models

  celltypist --update-models
  

  This will download the latest models from the remote server.

• 2.3. Overview of the models

  celltypist --show-models
  

• 2.4. Celltyping based on the input of count table

  See 1.5. for the format of the desired count matrix.

  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir
  

  You can add a different model to be used in the --model option. If the --model is not provided, CellTypist will by default use the Immune_All_Low.pkl model. The output directory will be set to the current working directory if --outdir is not specified.

  If your input file is in a gene-by-cell format (genes as rows and cells as columns), add the --transpose-input option.

  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir --transpose-input
  

  If the input is provided in the .mtx format, you will also need to specify the --gene-file and --cell-file options as the files containing names of genes and cells, respectively.

  The default mode (--mode best_match) for prediction is to choose the cell type with the largest score/probability as the final prediction; setting --mode prob_match combined with a probability threshold (default to 0.5, --p-thres 0.5) will enable a multi-label classification, which assigns 0 (i.e., unassigned), 1, or >=2 cell type labels to each query cell.

  Other options that control the output files of CellTypist include --prefix which adds a custom prefix and --xlsx which merges the output files into one xlsx table. Check celltypist --help for more details.

• 2.5. Celltyping based on AnnData

  See 1.6. for the requirement of the expression matrix in the AnnData object (.h5ad).

  celltypist --indata /path/to/input/adata --model Immune_All_Low.pkl --outdir /path/to/outdir
  

  Other command line options are the same as in 2.4..

• 2.6. Use a majority voting classifier combined with celltyping

  See 1.7. for how the majority voting classifier works.

  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir --majority-voting
  

  During the majority voting, to define cell-cell relations, CellTypist will use a heuristic over-clustering approach according to the size of the input data with the aid of a Leiden clustering pipeline. Users can also provide their own over-clustering result to the --over-clustering option. This option can be specified in several ways:

  1. an input plain file with the over-clustering result of one cell per line.
  2. a string key specifying an existing cell metadata column in the AnnData (pre-created by the user).
  3. if none of the above is provided, will use a heuristic over-clustering approach, noted above.

  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir --majority-voting --over-clustering /path/to/over_clustering/file
  

  There is also a --min-prop option (defaults to 0) which controls the minimum proportion of cells from the dominant cell type required to name a given subcluster by this cell type. Subcluster that fails to pass this proportion threshold will be assigned Heterogeneous.

  Other command line options are the same as in 2.4..

• 2.7. Generate visualisation figures for the results

  In addition to the tables output by CellTypist, you have the option to generate multiple figures to get an overview of your prediction results. See 1.5., 1.6. and 1.7. for what these figures represent.

  #Plot the results after the celltyping process.
  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir --plot-results
  #Plot the results after the celltyping and majority-voting processes.
  celltypist --indata /path/to/input/file --model Immune_All_Low.pkl --outdir /path/to/outdir --majority-voting --plot-results
  

3. Use in the R environment

Currently, there is no plan for R compatibility. Try to convert R objects into AnnData for use in CellTypist.

Supplemental guidance

• Generate a custom model

  As well as the models provided by CellTypist (see 1.2.), you can generate your own model from which the cell type labels can be transferred to another scRNA-seq dataset. This will be most useful when a large and comprehensive reference atlas is trained for future use, or when the similarity between two scRNA-seq datasets is under examination.

  Inputs for data training

  The inputs for CellTypist training comprise the gene expression data, the cell annotation details (i.e., cell type labels), and in some scenarios the genes used. To facilitate the training process, the train function (see below) has been designed to accommodate different kinds of input formats:

  1. The gene expression data can be provided as a path to the expression table (such as .csv and .mtx), or a path to the AnnData (.h5ad), with the former containing raw counts (in order to reduce the file size) while the latter containing log1p normalised expression (to 10,000 counts per cell) stored in .X or .raw.X. In addition to specifying the paths, you can provide any array-like objects (e.g., csr_matrix) or AnnData which are already loaded in memory (both should be in the log1p format). A cell-by-gene format (cells as rows and genes as columns) is required.
  2. The cell type labels can be supplied as a path to the file containing cell type label per line corresponding to the cells in gene expression data. Any list-like objects (such as a tuple or series) are also acceptable. If the gene expression data is input as an AnnData, you can also provide a column name from its cell metadata (.obs) which represents information of cell type labels.
  3. The genes will be automatically extracted if the gene expression data is provided as a table file, an AnnData or a DataFrame. Otherwise, you need to specify a path to the file containing one gene per line corresponding to the genes in the gene expression data. Any list-like objects (such as a tuple or series) are also acceptable.

  One-pass data training

  Derive a new model by training the data using the celltypist.train function:

  #Training a CellTypist model.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input)
  

  If the input is a table file, an AnnData or a DataFrame, genes will be automatically extracted and the genes tag can thus be omitted from the above code. If your input is in a gene-by-cell format (genes as rows and cells as columns), remember to pass in the transpose_input = True argument.

  Before the training is conducted, the gene expression format will be checked to make sure the input data is supplied as required. For example, the expression matrix should be in log1p normalised expression (to 10,000 counts per cell) if the input is an AnnData. This means when you subset the input with given genes (e.g., by highly variable genes), an error may be raised as CellTypist cannot judge the input as properly normalised with only a subset of genes. In such a case, pass in check_expression = False to skip the expression format check.

  #Training a CellTypist model with only subset of genes (e.g., highly variable genes).
  #Restricting the input to a subset of genes can accelerate the training process.
  #Use `AnnData` here as an example.
  new_model = celltypist.train(some_adata[:, some_adata.var.highly_variable], labels = label_input, check_expression = False)
  

  By default, data is trained using a traditional logistic regression classifier. This classifier is well suited to datasets of small or intermediate sizes (as an empirical estimate, <= 100k cells), and usually leads to an unbiased probability range with less parameter tuning. Among the training parameters, three important ones are solver which (if not specified by the user) is selected based on the size of the input data by CellTypist, C which sets the inverse of L2 regularisation strength, and max_iter which controls the maximum number of iterations before reaching the minimum of the cost function. Other (hyper)parameters from LogisticRegression are also applicable in the train function.

  When the dimensions of the input data are large, training may take longer time even with CPU parallelisation (achieved by the n_jobs argument). To reduce the training time as well as to add some randomness to the classifier's solution, a stochastic gradient descent (SGD) logistic regression classifier can be enabled by use_SGD = True.

  #Training a CellTypist model with SGD learning.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input, use_SGD = True)
  

  A logistic regression classifier with SGD learning reduces the training burden dramatically and has a comparable performance versus a traditional logistic regression classifier. A minor caveat is that more careful model parameter tuning may be needed if you want to utilise the probability values from the model for scoring cell types in the prediction step (the selection of the most likely cell type for each query cell is not influenced however). Among the training parameters, two important ones are alpha which sets the L2 regularisation strength and max_iter which controls the maximum number of iterations. Other (hyper)parameters from SGDClassifier are also applicable in the train function.

  When the training data contains a huge number of cells (for example >500k cells) or more randomness in selecting cells for training is needed, you may consider using the mini-batch version of the SGD logistic regression classifier by specifying use_SGD = True and mini_batch = True. As a result, in each epoch (default to 10 epochs, epochs = 10), cells are binned into equal-sized (the size is default to 1000, batch_size = 1000) random batches, and are trained in a batch-by-batch manner (default to 100 batches, batch_number = 100).

  #Get a CellTypist model with SGD mini-batch training.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input, use_SGD = True, mini_batch = True)
  

  By selecting part of cells for training (default to 1,000,000 cells with possible duplications, epochs x batch_size x batch_number), training time can be again reduced and the performance of the derived model is shown to persist as compared to the above two methods. Since some rare cell types may be undersampled during this procedure, you can pass in the balance_cell_type = True argument to sample rare cell types with a higher probability, ensuring close-to-even cell type distributions in mini-batches (subject to the maximum number of cells that can be provided by a given cell type).

  There are also some free texts that can be inserted (e.g., date) to describe the model. Check out the celltypist.train for more information.

  The resulting model is an instance of the Model class as in 1.4., and can be manipulated as with other CellTypist models.

  Save this model locally:

  #Write out the model.
  new_model.write('/path/to/local/folder/some_model_name.pkl')
  

  A suggested location for stashing the model is the models.models_path (see 1.2.). Through this, all models (including the models provided by CellTypist) will be in the same folder, and can be accessed in the same manner as in 1.4..

  #Write out the model in the `models.models_path` folder.
  new_model.write(f'{models.models_path}/some_model_name.pkl')
  

  To leverage this model, first load it by models.Model.load.

  new_model = models.Model.load('/path/to/local/folder/some_model_name.pkl')
  

  This model can be used as with the built-in CellTypist models, for example, it can be specified as the model argument in annotate.

  #Predict the identity of each input cell with the new model.
  predictions = celltypist.annotate(input_file, model = new_model)
  #Alternatively, just specify the model path (recommended as this ensures the model is intact every time it is loaded).
  predictions = celltypist.annotate(input_file, model = '/path/to/local/folder/some_model_name.pkl')
  #If the model is stored in `models.models_path`, only the model name is needed.
  predictions = celltypist.annotate(input_file, model = 'some_model_name.pkl')
  

  Downstream operations are the same as in 1.4., 1.5., 1.6., and 1.7..

  Two-pass data training incorporating feature selection

  Some scRNA-seq datasets may involve the noise mostly from genes not helpful or even detrimental to the characterisation of cell types. To mitigate this, celltypist.train has the option (feature_selection = True) to do a fast feature selection based on the feature importance (here, the absolute regression coefficients) using SGD learning. In short, top important genes (default: top_genes = 300) are selected from each cell type, and are further combined across cell types as the final feature set. The classifier is then re-run using the corresponding subset of the input data.

  #Two-pass data training with traditional logistic regression after SGD-based feature selection.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input, feature_selection = True)
  #Two-pass data training with SGD learning after feature selection.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input, use_SGD = True, feature_selection = True)
  #Two-pass data training with SGD mini-batch training after feature selection.
  new_model = celltypist.train(expression_input, labels = label_input, genes = gene_input, use_SGD = True, mini_batch = True, feature_selection = True)
  

  If you prefer other feature selection approaches and obtain a set of genes which are designated as important features, you can subset your input data and train the CellTypist model accordingly. As noted in the previous section, remember to pass in the check_expression = False argument.

  new_model = celltypist.train(expression_input_subset, labels = label_input, genes = gene_input, check_expression = False)
  

  The downstream workflow is the same as that from one-pass data training.

• Cross-species model conversion

  It is always recommended to predict a query dataset using the reference model from the same species. In cases where a cross-species label projection is needed, you can convert the model of interest to its "orthologous" form of another species. This is achieved by aligning orthologous genes between species.

  Load a human immune model.

  model = models.Model.load('Immune_All_Low.pkl')
  

  This model can be converted to a mouse equivalent through the convert method. By default, a human-mouse conversion (or the opposite) will be conducted by automatically detecting the species of the model (e.g., human) and transforming it to the other species (e.g., mouse).

  #Note `model` is modified in-place.
  model.convert()
  

  By default (unique_only = True), only 1:1 orthologs between the two species are kept and all other genes are discarded in the model. You can also keep those genes (including both 1:N and N:1 orthologs) by specifying unique_only = False. By doing so, you need to specify how these 1:N orthologs will be handled: for each gene, averaging the classifier weights (collapse = 'average', which is the default when unique_only = False) or randomly choosing one gene's weight as the representative (collapse = 'random') from all its orthologs.

  #For illustration purpose. Convert the model by utilising 1:N orthologs and their average weights.
  #model.convert(unique_only = False, collapse = 'average')
  

  As mentioned above, the default mode is a human-to-mouse (or mouse-to-human) conversion using the built-in gene mapping file (Ensembl105 version). For conversion to other species, you can provide a different file (map_file), with one column being the species of the model and the other column being the species you want to convert to. Check out models.Model.convert for more information.

  Lastly, write out the converted model locally.

  model.write('/path/to/local/folder/some_model_name.pkl')
  

  This model can be used as with other CellTypist models.