Towards the prediction of breast cancer cell behaviour in the lymphatics using in vitro and in silico methods

Breast cancer is the most common malignant cancer form in women and an estimated 318,000 new cases of breast cancer will be diagnosed in 2017. Metastasis accounts for 90% of cancer related deaths and the lymphatics serve as the primary route for the metastatic spread of breast cancer cells (BCCs). The dynamics by which BCCs travel in the lymphatics to distant sites, and eventually establish metastatic tumours, remain poorly understood. It has been shown that the microenvironment surrounding cancer cells plays an important role in determining their behaviour. Therefore, characterising the fluidic forces that BCCs are exposed to while travelling in the lymphatics could potentially reveal mechanisms that regulate BCC metastasis. The goal of this work was to develop a model capable of predicting the surface forces BCCs experience in the lymphatics by modelling their behaviour using numerical and experimental techniques. A microfluidic test facility was developed in which two types of BCCs, metastatic MDA-MB-231 cells and non-metastatic MCF-7 cells, were subjected to lymphatic flow rates (Re < 1) in a 100x100µm channel and their response to the flow, in terms of velocity and spatial distribution, was analysed. The behaviour of the BCCs (η = dp/W = 0.03-0.81, where dp is the particle diameter and W is the channel width) was compared to rigid particles of similar size (η = 0.05-0.32) to determine whether differences in the BCCs morphological properties lead to different transport mechanisms and ultimately different surface forces. A distinct difference between the behaviour of BCCs and particles was recorded. Parabolic velocity profiles were recorded for all particle sizes. All particles were found to lag the fluid velocity, the larger the particle the slower its velocity relative to the local flow (5-15% velocity lag recorded). The BCCs travelled ~40% slower than the undisturbed flow, indicating that morphology and size affects their response to lymphatic flow conditions. BCCs adhered together, forming aggregates (η = 0.3±0.07 and 0.35±0.05 for the MCF-7 and MDA-MB-231 cells respectively) whose behaviour was irregular. Single MCF-7 cells (η = 0.14±0.04) were distributed uniformly across the channel in comparison to single MDA-MB-231 cells (η = 0.18±0.04) which travelled in the central region (88% of BCCs found within 0.35W≤ y ≤0.64W), indicating that metastatic BCCs may be subjected to a lower range of flow induced surface forces. Numerical modelling techniques, using the Dynamic Fluid Body Interaction method, were employed to quantify the flow induced surface forces acting on BCCs in the lymphatics. An experimentally validated numerical model capable of predicting the advection of large particles (η = 0.1-0.4) in confined flow conditions, representative of lymphatic scales, was created. Both 2D and 3D simulations were carried out resulting in a total of 25 models. The simulations were in good agreement with the experiments (<12% difference) across the channel (0.2W≤ y ≤0.8W), with differences up to 25% in the near-wall region. The maximum shear stress experienced by the particles increased with increasing particle size and proximity to the wall. Particles experience a range of shear stresses (0.002-0.12Pa) and spatial shear gradients (maximum of 0.137Pa/μm) depending on their size and radial position. Literature indicates spatial shear stress gradients of 0.004Pa/μm are associated with BCC apoptosis. Knowledge of such values may provide indications for critical levels of surface forces that cause BCC membranes to react in a manner that can determine their metastatic potential. Gaining a better understanding of the complexities of the flow induced surface forces experienced by BCCs in lymphatic flows will improve prospects for developing effective breast cancer treatments.