Immunologic Effects of Red Blood Cell and Platelet Transfusions in Neonates

Abstract

Purpose of Review:

Premature neonates are frequently transfused red blood cells (RBC) or platelets to raise hemoglobin or platelet counts. However, these transfusions may have unintended effects on the immune system. This review will summarize the newest discoveries on the immunologic effects of RBC and platelet transfusions in neonates, and their potential impact on neonatal outcomes.

Recent Findings:

Neonatal RBC transfusions are associated with increases in plasma pro-inflammatory cytokines, but recent findings suggest sex-specific differential responses. At least one cytokine (MCP-1) rises in females receiving RBC transfusions, but not in males. These inflammatory responses correlate with poorer neurodevelopmental outcomes in heavily transfused female infants, while preterm male infants seem to be more sensitive to severe anemia. Platelet transfusions in preterm neonates are associated with increased neonatal mortality and morbidity. The underlying mechanisms are unknown, but likely related to the immune/inflammatory effects of transfused platelets. Adult platelets are different from neonatal platelets, with the potential to be more pro-inflammatory. Early pre-clinical data suggest that platelet transfusions alter the neonatal systemic inflammatory response and enhance immune cell migration.

Summary:

RBC and platelet transfusions alter neonatal immune and inflammatory responses. Their pro-inflammatory effects might worsen neonatal disease or affect neurodevelopmental outcomes.

INTRODUCTION

Anemia and thrombocytopenia are the most common hematological disorders in neonates, and red blood cells (RBC) and platelets are frequently transfused to premature infants admitted to the Neonatal Intensive Care Unit (NICU). In a recently published multicenter retrospective study of neonatal transfusion practices, 71% of neonates born at a gestational age (GA) <27 weeks received at least one RBC transfusion, and 34% received one or more platelet transfusions. These percentages decreased as GA increased (Table 1).¹ While RBC and platelet transfusions are given to increase the hemoglobin (Hb) or the platelet count (PC), respectively, there is growing evidence of off-target effects, particularly on the immune system. This review will focus on our current understanding of the immunologic effects of RBC and platelet transfusions in neonates.

Table 1.

Incidence of RBC and platelet transfusions in neonates with a hospital stay >3 days, per gestational age

Gestational Age (wks)	Number	Any RBC % (95% CI)	Any Platelet % (95% CI)
<27	295	71 (66–76)	34 (28–39)
27–28	277	45 (39–51)	11 (7–14)
29–32	987	13 (11–15)	5.8 (4.3–7.2)
33–36	3063	3.1 (2.5–3.7)	1.6 (1.2–2.0)
37+	8199	2.5 (2.2–2.8)	1.7 (1.4–2.0)

Adapted from Patel et al., J Peds, 2021.¹

RBC TRANSFUSIONS IN NEONATES

In current practice, RBC transfusions are given when the Hb falls below a somewhat arbitrary threshold, below which oxygen delivery to the tissues is thought to be compromised. Four large studies to date have compared the use of restrictive vs. liberal Hb thresholds for RBC transfusions in extremely-low-birth-weight infants (<1,000g at birth), with all reporting short-term (at discharge) and long-term neurodevelopmental outcomes. While the data from the initial studies was conflictive,^2,3 two larger recently published randomized trials (TOP and ETTNO) found no differences between the liberal and restrictive transfusion groups in the primary outcome of death or neurodevelopmental impairment at 24 months of age.^4,5** In contrast to prior observational studies, which had reported an association between RBC transfusions and both bronchopulmonary dysplasia (BPD) and necrotizing enterocolitis (NEC) (diseases of prematurity with a strong inflammatory component),⁶ none of the randomized trials found increased rates of those or other complications in neonates transfused more liberally.⁷ However, the neurodevelopmental outcomes evaluated in the RCTs measured severe deficits, and it remains unclear whether RBC transfusions (or anemia) are associated with more subtle neurodevelopmental problems (i.e. attention deficits) that manifest later in life. A school-age follow up study of infants in the TOP trial is in progress, and will answer this question.

RBC transfusion-related immunomodulation in neonates

RBC transfusion related immunomodulation (TRIM) has been widely studied in critically ill adult and pediatric patients, with only few studies in neonates. TRIM can manifest with either immunosuppressive or pro-inflammatory effects in the recipient, likely due to the dynamic and complex immune response occurring in the critically ill. While the underlying mechanisms are not yet fully elucidated, a “two hit” model is likely, where an initial inflammatory process sensitizes the host neutrophils that then respond to biological mediators that accumulate in the RBC transfusion product.^8,9 As such, differences in the host characteristics, transfusion timing, donor characteristics, and blood processing/storage all have the potential to contribute to the differential effects of TRIM.⁸ However, it is unclear how RBC transfusions interact with the developmentally unique immune system of the premature neonate (Table 2), which is physiologically unprepared for the extrauterine environment.

Table 2.

Key developmental differences between the neonatal and adult innate and adaptive immune systems

	Neonatal compared to the Adult Immune System
Innate Immune System	• First immune system to mature, active during embryonic stage (0–8 weeks post conception)
Monocytes/Macrophages	• Poor cytokine production (TNF-α, IL-1β and IL-12) well into infancy • Reduced co-stimulatory cell surface molecules (CD86 and CD40) • Poorly responsive to IFN-γ and CD40L • Decreased migration • Impaired phagocytosis and antigen presentation to T-cells • Elevated levels of IL-27 (immunosuppressive) • Reduced TLR-4 activation with TRIF-dependent signaling (compared to MyD88-dependent signaling in adults)
Neutrophils	• Elevated cycling rates • Reduced transendothelial migration, endothelial adherence, and chemotaxis  ○ Lower levels of L-selectin, β2 integrin CD18/CD11b, and CD18/CD11a • Comparable phagocytic and microbicidal activity in healthy term neonates  ○ Reduced in sick and/or preterm neonates • Delayed apoptosis
Complement	• Reduced complement enzyme levels that reach ~50% adult levels at term • Reliance on alternate/lectin pathways at birth
Adaptive Immune System	• Begins to function by second trimester but does not reach full capacity by term
T-Lymphocytes (T-cells)	• Increased CD45RA+ T-cells (immunologically naïve state) • Higher levels of regulatory T-cells (expressing CD25 and FoxP3) • Decreased production of IL-4 and IL-5 • Limited cytotoxic activity
B-Lymphocytes (B-cells)	• Lower levels of immunoglobulin production  ○ Adult levels of IgG, IgA, and IgE not reached until several months-years after birth
Cytokines	• Similar levels of IL-1 • Variable levels of IL-2, IL-10 and IL-6 • Decreased levels of IL-4, IL-8, IL-10, IL-12, TNF-α and IFN-γ • Decreased cytokine receptor responsiveness

IFN-γ = interferon gamma; TLR-4 = toll like receptor 4; LPS = lipopolysaccharide; TRIF = TIR-domain-containing adapter-inducing interferon-β; FoxP3 = forkhead box P3; TNF-α = tumor necrosis factor-α

To investigate the potential effects of RBC transfusions on neonatal inflammation, investigators measured plasma inflammatory cytokines and markers of endothelial activation before and after RBC transfusion in premature infants.^10–12 While the specific cytokines and endothelial activation patterns differed among studies (likely due to differences in unit storage, additives, and cytokine measurement timing), all found an association between RBC transfusions and elevated markers of endothelial activation, pro-inflammatory cytokines, or both (Table 3). Repeat RBC transfusions, to which most preterm infants are exposed, have also been associated with incremental increases in plasma pro-inflammatory cytokines.¹³ Most recently, Benavides et al. serially measured a panel of plasma cytokines in a subset of extremely preterm infants randomized to liberal or restrictive transfusion thresholds as part of the TOP study, and found nine pro-inflammatory cytokines that increased significantly in proportion to the number of transfusions received.¹⁴** Taken together, the available studies suggest that RBC transfusions have systemic pro-inflammatory effects in neonates, although the mechanisms mediating these effects are unclear.

Table 3.

Studies reporting on factors in neonatal serum before and after RBC transfusion.

Study	Population	Storage time	Factors Measured	Factors present in RBC blood product	Timing of serum measurements	Factors elevated in serum after RBC
Locke, 200510	24–32 wks GA >24hrs old and < 37 wks PMA	42 days maximum	IL-6, IL-10, TNF-α, IL-1β	IL-1β	6 hrs prior to Tx 1 hr post Tx	None
Keir, 201211	≤ 28 wks GA 2 to 6 wks PMA	Average 23 days (range 6–33 days)	GM-CSF, IFN-γ, IL-1β, IL-2, IL-4, IL-5, IL-6, IL-7, IL-8, IL-10, IL-12, IL-13, MCP-1, TNF-α	IL-5, IL-6, IL-8, IL-10, IL-13, MCP-1, TNF-α, sICAM-1, sVCAM-1, MIF	Immediately prior to Tx 2–4 hrs post Tx	IL-1β, TNF-α, IL-8, MCP-1, MIF, sICAM-1
Dani, 201712	≤ 32 wks GA > 7 days PMA	< 7 days	IL-1β, IL-6, IL-8, TNF-α, IFN-γ, IL-17, MCP-1, IP-10, sICAM-1, sVCAM	Not measured	2 hrs prior to Tx 2h, 24h, and 48h post Tx	IL-1β, IL-8, IFN-γ, IL-17, MCP-1, IP-10, sICAM-1

RBC = red blood cell; GA = gestational age; PMA = post menstrual age; Tx = transfusion; TNF-α = tumor necrosis factor-α; GM-CSF = granulocyte-macrophage colony stimulating factor; sICAM-1 = soluble intercellular adhesion molecule-1; sVCAM-1 = soluble vascular cell adhesion molecule-1; MIF = macrophage inhibitory factor; MCP-1 = monocyte chemoattractant protein-1; TNF-α = tumor necrosis factor-α;

Sex-specific inflammatory responses to RBC transfusions in neonates

In the study by Benavides et al., one of the measured cytokines (monocyte chemoattractant protein-1 [MCP-1, CCL2]), changed in a sex-specific manner, rising in females with each additional transfusion, but not in males. Furthermore, higher concentrations of MCP-1 were associated with worse cognitive and motor outcomes at 12 months of age.¹⁴** This study was important in that it provided the first evidence of sex-specific inflammatory responses to RBC transfusions in preterm infants, and linked neonatal transfusion-related inflammation to later cognitive outcomes. These findings were also consistent with the observation that female preterm infants randomized to liberal transfusion thresholds in a prior trial (which were much higher than the liberal Hb thresholds used in the more recent trials)² had worse neurodevelopmental outcomes and significantly decreased cerebral white matter volume in adolescence, compared to girls who had been managed with more restrictive thresholds.^15–17* Together, these studies suggest that RBC transfusions have a pro-inflammatory effect that is more pronounced in premature female infants, with the potential to affect long term neurodevelopmental outcomes. Interestingly, among male preterm infants randomized to liberal or restrictive transfusion thresholds (in the same subset of TOP infants), lower pre-transfusion Hb levels were associated with lower cerebral white matter volume at discharge, suggesting that the male brain might be more sensitive to anemia while the female brain may be more sensitive to the pro-inflammatory effects of transfusions.¹⁸

Red Blood Cell Transfusion and Necrotizing Enterocolitis (NEC)

NEC is a serious complication of prematurity¹⁹ and multiple observational studies have implicated anemia, RBC transfusions, or both in its pathogenesis. However, the data regarding the relative importance of RBC transfusions (vs. anemia) has been conflicting,^20–23 and none of the four large randomized trials of neonatal RBC transfusion thresholds found differences in the incidence of NEC between neonates transfused at restrictive vs. liberal thresholds.^2–5

Nevertheless, fecal calprotectin (FC), a marker of intestinal inflammation, has been found to increase with RBC transfusions in preterm neonates, reaching particularly high levels in infants who received multiple transfusions, had more severe anemia, or were transfused with RBCs stored >21 days.²⁴ Animal studies have investigated the effects of anemia and/or RBC transfusions on neonatal intestinal vasoreactivity and inflammation, as potential mechanisms mediating intestinal injury. RBC transfusion in preterm lambs disrupted mesenteric vascular tone by decreasing arterial eNOS,²⁵ consistent with clinical studies using NIRS that showed fluctuations and decreases in mesenteric oxygen patterns in infants who developed NEC post-transfusion compared to infants who were transfused and did not develop NEC.²⁶ In a murine model of neonatal phlebotomy induced anemia (PIA), significant gut hypoxia extending beyond the epithelial layer and into the lamina propria was observed in anemic pups. Hypoxia-exposed macrophages in the intestinal lamina propria exhibited up-regulated HIF-1α expression and IFNɣ production, which led to reduced tight junction protein ZO-1 expression and increased intestinal barrier permeability.²⁷ Similarly, in a separate study of newborn mice with more severe PIA, anemic pups (with or without RBC transfusion) exhibited intestinal macrophage infiltration and increased barrier permeability. However, only pups who were both anemic and transfused had increased macrophage activation and histologic evidence of gut injury, suggesting that both the degree of anemia (a possible priming step) and RBC transfusion (a possible activation step) play a role in the development of NEC. Mechanistically, the intestinal macrophages in anemic pups were activated by free hemoglobin released from RBCs during storage (7 days), through a TLR-4 mediated mechanism.²⁸** In both of these studies, macrophage depletion with clodronate liposomes was protective against neonatal intestinal injury, highlighting the critical role of macrophages in the pathogenesis of NEC.

Influence of Component and Donor Characteristics

Studies have evaluated the impact of different component and donor characteristics on neonatal outcomes. A study randomizing preterm neonates to receiving only fresh RBC units (stored <7 days) versus standard practices found no differences in any neonatal outcome studied, including NEC and BPD.²⁹ More recently, preterm infants were randomized to transfusions with washed or unwashed (standard) leukoreduced RBCs, and pro-inflammatory cytokines and markers of endothelial activation were measured immediately prior to and 4–6 hours after each of the first three transfusions received.³⁰* After the third transfusion, infants who received standard RBCs had elevated IL-17A and TNF, compared to pre-transfusion levels, while infants transfused with washed blood showed reduced IL-17A, TNF, IL-6, IL-8, and IFN-γ levels, suggesting that the pro-inflammatory response to transfusion can be attenuated with washing, presumably by removing the free hemoglobin and other inflammatory mediators present in the supernatant of stored RBCs. Intriguingly, the sex and age of the donor also seem to affect neonatal outcomes, with preterm infants transfused exclusively with RBCs from female donors having better outcomes than neonates transfused with RBCs from male donors.³¹* The protective association of transfusions from female donors increased with advancing donor age, but decreased as the number of transfusions increased (likely reflecting increased severity of illness). The mechanisms mediating these associations are unknown.

PLATELET TRANSFUSIONS IN NEONATES

Most platelet transfusions in the NICU are given to non-bleeding neonates whose PCs fall below a threshold at which the bleeding risk is thought to increase. Because preterm neonates have a high incidence of spontaneous intracranial bleeding, most commonly intraventricular hemorrhages (IVHs), it has historically been accepted that neonates should be transfused at higher PC thresholds than children or adults, although there is little evidence to support this. Two large randomized trials have compared liberal vs. restrictive platelet transfusion thresholds in this population: The first, published in 1993, preterm neonates in the first week of life (the period when nearly all IVHs occur) to receive a platelet transfusion when the PC fell below 150×10⁹/L or 50×10⁹/L. This study found no differences in the incidence or severity of IVH between the groups.³² The much larger PlaNeT-2 study, published in 2019, randomized preterm neonates to receive platelet transfusions for PCs <50 ×10⁹/L or <25×10⁹/L. 90% of neonates randomized to the liberal group received at least one platelet transfusion, compared to 53% in the restrictive group. Surprisingly, the liberal transfusion group exhibited a significantly higher incidence of death or major bleeding in the 28 days following randomization (the primary outcome) and BPD, compared to the restrictive group.³³** The mechanisms mediating these findings are unknown, but are likely related to the immune/inflammatory effects of platelet transfusions.

Platelets as Immune Cells

While platelets are the primary cellular component of hemostasis, it has become increasingly clear that they sit at the intersection between hemostasis and immunity and should also be considered immune cells, a concept further supported by the discovery of immune subpopulations of megakaryocytes at all stages of development.^34,35** Consistent with the importance of the platelets’ immune functions, the most enriched biological processes in the transcriptome of neonatal and adult platelets are related to immune functions, rather than hemostasis or coagulation.³⁶

Platelets can interact with immune and endothelial cells directly through their surface receptors, or indirectly through the regulated release of their granular content, newly produced proteins, or platelet microparticles. Upon activation, platelets translocate P-selectin from the alpha granules to the platelet surface, where it is available to bind to PSGL-1, found on multiple cell types including neutrophils and monocytes. This cell-cell interaction triggers immune cell activation, the release of cytokines and chemokines by leukocytes and endothelial cells, facilitates cellular migration into tissues, and stimulates the formation and release of neutrophil extracellular traps (NETs).^37–39 In addition, platelets carry growth factors, cytokines, chemokines and critical immune modulators, such as CD40L (Table 4), which are released upon activation either in free form or in platelet microparticles. These factors modulate both innate and adaptive immune responses, in a highly stimulus- and context-dependent manner (Reviewed in Maouia et al., 2020).⁴⁰ The effects of platelets on both innate and adaptive immunity are complex and more nuanced than previously suspected, with platelets exhibiting either pro-inflammatory or anti-inflammatory/immune-regulatory effects, depending on the disease context.⁴⁰

Table 4.

Main inflammatory and immune modulatory factors released by activated human platelets.

CLASS OF MEDIATOR	FACTOR	STORED OR SYNTHESIZED	TARGET IMMUNE CELLS
INFLAMMATORY MODULATORS	Histamine	Stored	ECs, monocytes, PMNs, NK cells, T and B cells, eosinophils
	Serotonin (5-HT)	Stored	Monocytes, macrophages, DCs
INFLAMMATORY AND IMMUNOMODULATORY LIPIDS	TXA2	Synthesized	Platelets, T lymphocytes and macrophage subsets
	PAF	Synthesized	Platelets, PMNs, monocytes, macrophage and lymphocyte subsets
IMMUNE MODULATORS	CD40L (CD154)	Stored	ECs, monocytes, DC subtypes, B and T lymphocytes
GROWTH FACTORS	PDGF	Stored	Monocytes, macrophages, T lymphocytes
	TGF-β	Stored	Monocytes, macrophages, T and B lymphocytes
CHEMOKINES	PF4 (CXCL4)	Stored	PMNs, monocytes, macrophages
	NAP-2 (CXCL7)	Proteolytic cleavage of stored precursors	PMNs
	GRO-α (CXCL1)	Stored	PMNs
	ENA-78 (CXCL5)	Stored	PMNs
	SDF-1 (CXCL12)	Stored	Bone marrow derived progenitors
	RANTES (CCL5)	Stored	Monocytes, eosinophils, basophils, NK cells, T lymphocyte and DC subsets
	MIP-1α (CCL3)	Stored	Monocytes, eosinophils, basophils, NK cells, lymphocyte and DC subsets
	MCP-3 (CCL7)	Stored	Monocytes, basophils, NK cells, lymphocyte and DC subsets
	Granzyme A	Stored	Monocytes
CYTOKINES	IL-1β	Synthesized	Monocytes, DC and macrophage subsets, T-cell lines, ECs,
	IL-1α	Stored (?)	Same as IL-1β
	HMGB1	Stored	Macrophages, PMNs, ECs
	GM-CSF	Stored	Eosinophils
PLATELET AUTOCRINE FACTOR	Triggering receptor expressed on myeloid cells – like transcript-1 (TLT-1)	Stored	Platelets

Adapted from Manne et al., Platelets, 2017.⁵³

Developmental differences between adult and neonatal platelets

When transfusing platelets into neonates, it is important to consider the substantial biological differences that exist between neonates and adults in regard to every organ system, including platelet function.⁴¹ Human neonatal platelets are hyporeactive in response to most agonists and exhibit a degranulation defect and less P-selectin expression upon activation compared to adult platelets.^42,43 These differences are more pronounced in preterm neonates, who form less platelet-neutrophil aggregates than term neonates following platelet activation.⁴⁴ Paradoxically, despite the neonatal platelet hyporeactivity, whole blood tests of primary hemostasis show faster clotting in neonates than in adults, due to the presence of factors in neonatal blood that promote clotting, including a high hematocrit, high MCV, and high concentrations of vWF with a predominance of ultralong vWF polymers.⁴³ Thus, the platelet hyporeactivity is not a developmental deficiency, but rather an integral part of a different, but well-balanced, neonatal hemostatic system.

Developmental differences between neonatal and adult platelets in regard to immune functions have not been well characterized, but the presence of less surface P-selectin in activated neonatal platelets would predict a reduced ability to interact with immune cells compared to adult platelets. Studies comparing the human neonatal and adult platelet transcriptome and proteome suggest that neonatal platelets might have decreased immune/inflammatory potential. In a transcriptomic study, mRNAs related to protein synthesis, trafficking and degradation were up-regulated in neonatal platelets, while transcripts related to calcium transport/metabolism, actin cytoskeleton reorganization and cell signaling were down-regulated (consistent with their hypo-reactivity). While no significant differences were found in immune pathways globally, neonatal platelets had lower expression of CXCL5, a critical neutrophil chemoattractant (see Table 4).³⁶ In a mass spectrometry study of neonatal and adult platelets, proteins related to the inflammatory response, platelet activation, coagulation and complement activation were down-regulated in neonatal compared to adult platelets.⁴⁵ Neonates also have a markedly reduced ability to form NETs compared to adults, due to a NET-inhibitory factor produced by the placenta and present in neonatal blood for 3–14 days after birth.^46,47** It is likely that these features are all part of a developmentally unique immune system, similar to the neonatal hemostatic system, that suits the needs of the fetus (who lives in a nearly sterile environment) and the neonate.

Hemostatic and Immunologic Effects of Platelet Transfusions in Neonates

Most studies assessing the responses of neonates to platelet transfusions have focused on the clinical and hemostatic effects. In vitro, a study mixing adult platelets with thrombocytopenic cord blood (a model of platelet transfusions) showed a significant shortening of closure times in response to epinephrine and collagen using a Platelet Function Analyzer, to ranges associated with thrombotic risk.⁴⁸ This suggested that the presence of adult platelets in neonatal blood could alter the hemostatic balance towards a pro-thrombotic phenotype, and provided proof-of-concept for a potential “developmental mismatch” associated with platelet transfusions in neonates.

Whether a similar mismatch occurs in immune/inflammatory responses is unknown, but recent studies in newborn mice have begun to explore this hypothesis. In a neonatal model of polymicrobial sepsis (using the cecal slurry model), cecal slurry batches differed substantially in their bacterial composition and sepsis-related mortality. Interestingly, platelet transfusions (from adult donors) increased the mortality and the systemic inflammatory response (plasma cytokines) of post-natal day 10 (P10) newborn mice infected with a low-mortality batch, but attenuated the inflammatory response and decreased the mortality of newborn mice infected with a high-mortality batch.⁴⁹* These findings demonstrated that platelet transfusions can have either pro-inflammatory or anti-inflammatory effects in sepsis, depending on the type and/or severity of the infectious stimulus. This was consistent with findings from an epidemiological study in critically ill pediatric patients, in which the effects of platelet transfusions on mortality were different depending on the severity of illness of the recipient.⁵⁰ In that regard, it is important to keep in mind that P10 mice developmentally might be closer to full-term rather than preterm human neonates, in whom clinical studies have consistently found an association between platelet transfusions and increased mortality and morbidity.

Two additional unpublished studies presented at the 2021 American Society of Hematology investigated the inflammatory effects of platelet transfusions in newborn mice. In healthy P10 mice, platelet transfusions induced a rapid transient elevation in IL-6, CXCL-1, G-CSF, and MCP-1. Similarly, P10 mice transfused with adult platelets following a non-lethal LPS dose exhibited higher IL-6, CXCL-1 and IL-10 levels 18 hours after LPS, compared to non-transfused mice.⁵¹ These observations suggested that transfusion of adult platelets into newborn mice could trigger or prolong an inflammatory response, although the contribution of platelet developmental differences to these findings was not demonstrated. Dr. Craig Morrell’s laboratory investigated the effect of adult versus neonatal platelets on monocyte inflammation and trafficking patterns. Using an in vitro co-culture of murine adult bone marrow monocytes and either neonatal or adult platelets, they found that platelets from both sources up-regulated inflammatory mRNAs in monocytes (NOS2, CXCL1, CCL2), but only co-culture with adult platelets increased CCR2 mRNA, which encodes the receptor for CCL2 (MCP-1) and regulates monocyte trafficking. Functionally, monocytes treated with adult, but not neonatal, platelets exhibited greater migration toward CCL2 in vitro and in vivo, which decreased upon blocking P-selectin.⁵²*

Conclusion

Taken together, these findings support the hypothesis that the transfusion of adult platelets into a neonate may alter the neonatal systemic immune response and enhance immune cell migration, at least in part as a result of the developmental differences in P-selectin expression. However, the specific mechanisms mediating these observations, and whether/how these (and other yet to be discovered) effects of platelet transfusions on neonatal immune/inflammatory responses contribute to the increased morbidity and mortality observed in transfused neonates, remain open questions.

Key Points:

• Neonates frequently receive transfusions of red blood cells (RBC) and platelets.

• Neonatal RBC transfusions are associated with elevations in plasma pro-inflammatory cytokines and recent studies suggest sex-specific differential responses, with some cytokines rising in female but not male infants in response to transfusion.

• Both the degree of anemia and the RBC transfusion likely play a role in the development of transfusion-associated necrotizing enterocolitis. In animal models of neonatal anemia and transfusion, the intestinal injury is mediated by activated intestinal macrophages.

• Platelet transfusions are associated with increased neonatal morbidity and mortality, likely related to the non-hemostatic effects of transfused adult platelets.

• Adult platelets are functionally different from neonatal platelets, and have the potential to be more pro-inflammatory. Early pre-clinical studies suggest that transfusion of adult platelets may alter the neonatal inflammatory response and enhance immune cell migration.