Storage-Induced Micro-Erythrocytes Are Rapidly Cleared from Recipient Circulation and Predict Transfusion Recovery

Background

Hypothermic storage of red blood cell (RBC) concentrates for up to 42 days is associated with biochemical, molecular, morphological, and mechanical modifications. This "storage lesion" increases with storage duration and is associated with increased clearance of transfused storage-damaged RBCs from the recipient's circulation in the first few hours post-transfusion. This rapid clearance reduces transfusion efficacy, but how it occurs is not fully elucidated. RBCs with reduced surface area called "storage-induced micro-erythrocytes" (SMEs) were recently described. Their proportion increases from 2% to 23% during storage. Their reduced surface-to-volume ratio is expected to induce rapid mechanical clearance by the spleen. We aimed to evaluate whether SMEs can be used as a marker of transfusion efficacy, if this subpopulation of RBCs is preferentially cleared by the spleen after transfusion, and if so, by which mechanisms.

Methods

We evaluated the proportion of SMEs in stored RBC concentrates in vitro using ImageStream and correlated it to the ⁵¹Chromium-labeled 24h post-transfusion recovery (24hPTR) in vivo in 31 healthy human volunteers. We then investigated the fate of SMEs during 8 ex vivo perfusions of human spleens (16 RBC concentrates stored for 35-42 days). Finally, we developed a mouse transfusion model to assess the fate of SMEs in vivo and determine their main mechanisms of clearance.

Results

The proportion of SMEs in RBC concentrates at day 42 of storage correlated negatively with 24hPTR in healthy volunteers (r=-0.42, P<0.01). When perfused ex vivo into human spleens, 15% of stored RBCs (35-42 days of storage) were cleared during the first 40 min of perfusion in a 2-step process: 7% of circulating RBCs disappeared in the first 2 min (1-2 passages through the spleen) while 8% were cleared between 10 and 40 min after initiating perfusion (>5 passages through the spleen). The percentage of SMEs correlated with splenic retention rate ex vivo (r=0.46, p<0.05). Morphological analysis of 6 stored RBC concentrates showed a mean decrease in the proportion of SMEs from 20.2% to 7.8% between the beginning and end of splenic perfusions.

In our mouse transfusion model, SMEs accumulated during RBC storage. The 24hPTR also decreased with storage duration (64% on Day 14 vs. 95% on Day 1). The decrease in 24hPTR of long-stored RBCs was mostly due to clearance of the SME subpopulation. SME and morphologically normal long-stored RBC subpopulations displayed clearances of 83% and 13%, respectively. Stored RBCs accumulated predominantly in the spleen post-transfusion, and were mainly ingested by macrophages. In macrophage-depleted mice, 24hPTR improved (from 64% to 79%), splenic accumulation and clearance of SMEs were delayed, and the proportion of inflammatory monocytes increased and mediated clearance. In splenectomized mice, clearance of SMEs was not delayed, but increased accumulation was observed in the liver and bone marrow, and increased erythrophagocytosis by inflammatory monocytes was also observed.

Conclusions

We show that the proportion of SMEs correlates with 24hPTR in healthy human volunteers and with retention in human spleens perfused ex vivo. In vivo mouse data confirms these findings, showing that SMEs are cleared from the recipient circulation during the 24h following transfusion. Clearance of SMEs is delayed in macrophage-depleted mice, suggesting a central role of macrophages in this process. The human spleen is also likely to clear SMEs from the recipient's circulation, as suggested by experiments with human spleens perfused ex vivo. However, the spleen is not required, because SME clearance is not affected in splenectomized mice. This suggests that other organs may compensate to remove SMEs and highlights the importance of eliminating these morphologically-altered RBCs.

Finally, quantification of SMEs is an operator-independent, reproducible marker of transfusion efficacy. It can be used to assess the potential of new processes to prepare and store RBC concentrates. Pre-transfusion quantification of SMEs could benefit chronically transfused patients, for whom improved transfusion efficacy is expected to reduce transfusion-induced iron overload.