Cell-Mediated Immunity (CMI) as the Instrument to Assess the Response Against the Allograft: Present and Future

The concept of Cell-Mediated Immunity (CMI) monitoring in transplantation has gained popularity over time and is now a reality. Significant technological advances have enabled us to test for multiple molecules and cells implicated in inflammatory or suppressive reactions to the graft. The main challenge nowadays is whether clinicians can use the information provided by the measurement of such markers to predict post-transplant outcome. To date a wide range of markers have been identified as promising biomarkers in the monitoring of individual responses to immunosuppression or in the determination of patient alloreactivity to the graft, which could prove helpful in the assessment of the occurrence of an adverse/side effect. Before these biomarkers are deemed suitable, standardisation of the methodology and validation of its feasibility in clinical outcome remains an ongoing challenge. The research community is currently facing a large effort towards the implementation of a standard methodology that is both highly reproducible and can reduce inter-laboratory variability, therefore generating consistency with data. The aim of this manuscript is to review the current literature regarding CMI monitoring in the field of solid organ transplantation (SOT), undertaking a comprehensive study of the latest findings. In addition, based upon current literature, we aim to propose a comprehensive classification of biomarkers to further aid our current understanding, taking in to account the type of transplantation, when its measurement should be applied and which would be the most suitable biomarker to assess.