Characterisation of dendritic cell immune profile in models of transfusion

Abstract

Blood transfusion modulates recipients’ immune responses sometimes resulting in poor clinical outcomes. Further, transfusion with blood products at date-of-expiry has been associated with exacerbation of this concerning problem. In-vitro studies have demonstrated exposure to packed red blood cell (PRBC) and platelet products modulate the responses of leukocytes, however, there are limited studies on the impact of transfusion on dendritic cells (DC). Despite DC being central to the immune response, their role in transfusion-related immune modulation remains largely undefined. I hypothesised that exposure to blood components changes DC phenotype and function, which could be one mechanism underpinning transfusion-related immune modulation. The potential for blood components (PRBC, buffy-coat-derived platelet concentrates (PC) and cryopreserved platelets (cryo-PLT)) to modulate responses of myeloid dendritic cells (mDC) and the specialised subset blood DC antigen 3 (BDCA3+) DC, was investigated. Amongst blood DC, BDCA3+ DC are the sole subset equipped with C-type lectin domain family 9 member A (Clec9A) receptor. Of interest to transfusion, Clec9A has been reported to bind filamentous actin (F-actin) when exposed, in artificially aged or modified red blood cells (RBC) and platelets. I further hypothesised Clec9A expressed on BDCA3+ DC has a role in transfusion-related immune modulation. Importantly, BDCA3+ DC are capable of cross-presentation of endogenous antigen via major histocompatibility complex (MHC) class I and have pathogen recognition receptors skewed towards recognition of viruses, unlike most mDC. I investigated the suitability of the fluorescein conjugated function-spacer-lipid (FSL-FLRO4) construct for labelling PRBC. I demonstrated that FSL-FLRO4 is a suitable tool for labelling PRBC at different storage duration and can be retained until date-of-expiry, aiding in-vitro visualisation and tracking of PRBC during routine storage. This was utilised in an assessment of erythrophagocytosis in my model of PRBC transfusion (see below). In addition, I investigated the unexpected loss of Clec9A surface expression on BDCA3+ DC observed during optimisation of the in-vitro transfusion model. Interestingly, I found loss of Clec9A was an effect of ethylenediaminetetraacetic acid (EDTA), as well as incubation temperature and duration. Therefore, recombinant human (rh)Clec9A was used to investigate a role for Clec9A in the context of transfusion, specifically whether this receptor bound fresh and/or stored PRBC, PC and cryo-PLT. Concurrently, I investigated whether blood components exposing F-actin for Clec9A ligation were present within the blood product. rhClec9A did not bind fresh or stored PRBC, and this outcome was supported by the lack of detection of F-actin. Although, no binding of rhClec9A to PC or cryo-PLT was demonstrated, F-actin was detected on both types of platelet products. As binding of rhClec9A to PRBC, PC or cryo-PLT was not detected, EDTA blood collection tubes were used for the remainder of the study under the premise that lack of expression of this receptor would not impact on outcomes in my transfusion models. I assessed the impact of fresh and/or stored blood products on mDC and BDCA3+ DC surface antigen and inflammatory profile using a human in-vitro whole blood model of transfusion. In parallel, to model the processes associated with viral or bacterial infection, polyinosinic:polycytidylic acid (polyI:C) or lipopolysaccharide (LPS) was added respectively. Exposure to PRBC and PC predominately suppressed surface antigen expression (CD40, CD80, CD83 and CD86) and inflammatory mediator production (interleukin (IL)-6, IL-8, IL-12, tumour necrosis factor-α and interferon-gamma inducible protein-10 or IL-10) on both DC subsets. These changes were often more evident in the presence of polyI:C and LPS, and when DC were exposed to stored PRBC. Similar modulation was evident for BDCA3+ DC when exposed to cryo-PLT alone and cryo-PLT in the presence of polyI:C and LPS. The impact of blood transfusion on the overall inflammatory response by leukocytes was also examined. The immunomodulatory effect of transfusion in-vitro was more pronounced in the presence of polyI:C and LPS. For PRBC, an additional erythrophagocytosis assay was conducted where the uptake of stored PRBC by mDC and BDCA3+ DC was significantly increased in comparison to fresher PRBC. This study provided the first evidence that exposure of PRBC, PC and cryo-PLT modulates mDC and/or BDCA3+ DC maturation and activation. The changes were more pronounced when modelling processes associated with concurrent viral or bacterial infection. Experimental evidence suggested that modulation of BDCA3+ DC was independent of Clec9A-F-actin interaction. My study highlights the importance of considering mechanisms associated with transfusion-related immune modulation in specific cell subsets – with changes in cell responses in rare but functionally important cell subsets overlooked when only assessing the overall leukocyte inflammatory response, which largely represents the response of the more abundant leukocyte populations. My results add to our knowledge of the changes in immune profiles that could be predicted in transfusion patients, in particularly those with infectious complications. My PhD provided detailed evidence of changes in DC phenotype and function in a model of blood transfusion, and I propose these changes are one mechanism underpinning transfusion-related immune modulation.