Speaker: Dr Boris Hejblum, ISPED, University of Bordeaux

Abstract: Flow cytometry is a high-throughput technology used to quantify multiple surface and intracellular markers at the level of a single cell.  This enables to identify cell sub-types, and to determine their relative proportions. Improvements of this technology allow to describe millions of individual cells from a blood sample using multiple markers.  This results in large datasets, whose manual analysis is highly time-consuming and poorly reproducible.  While several methods have been developed to perform automatic recognition of cell populations, most of them treat and analyze each sample independently.  However, in practice, individual samples are rarely independent (e.g. longitudinal studies).  Here, we propose to use a Bayesian nonparametric approach with Dirichlet process mixture (DPM) of multivariate skew t-distributions to perform unsupervised model-based clustering of flow-cytometry data. DPM models directly estimate the number of cell populations from the data, avoiding model selection issues, and skew t-distributions provides robustness to outliers and non-elliptical shape of cell populations.  To accommodate repeated measurements, we propose a sequential strategy relying on a parametric approximation of the posterior.  We illustrate the good performance of our method on simulated data, on an experimental benchmark dataset, and on new longitudinal data from the DALIA-1 trial which evaluates a therapeutic vaccine against HIV.  On the benchmark dataset, the sequential strategy outperforms all other methods evaluated, and similarly, leads to improved performance on the DALIA-1 data.  We have implemented an efficient partially collapsed Gibbs sampler with a Metropolis-Hastings step using slice-sampling to estimate the posterior partition of the data, available from CRAN in the R package NPflow.