NK Receptor (NKR)-Mediated Signaling Pathways in Cord Blood (CB) CD56^dim Versus Peripheral Blood (PB) CD56^dim NK Cells: Implication for Immaturity in Cord Blood NK CD56^dim Innate Immunity

Background: NK cells are characterized by absent CD3 but expression of CD56^dim (90%, cytotoxic) and CD56^bright (10%, mediator). NK cells may contribute to the immaturity in cord blood innate and adaptive immunity, and play an important role in the GVL effect post CBT. However, little is known regarding the NKR signaling pathways in CB vs PB CD56^dim NK cells and its relationship to the cytotoxic activity. We previously demonstrated the ability to ex-vivo expand CB into NK subsets with profound NK in-vitro and in-vivo cytotoxic activity (Ayello/Cairo BBMT 2006). We further observed that there were 33 and 37 proteins over and under expressed by proteomic expression profiling studies of CB vs PB CD56^dim (Shereck/Cairo, ASH 2007; ASPHO 2007; AACR 2007). The differential protein expressions included NKG2A, IP3R type 3, NCR3, MAPKAPK5, Notch 2, PLEK, and NF-X1 using both immunophenotype and proteomic profiling studies.

Objectives: To understand the importance of NKR signaling pathways in mediating the differential protein expression and thus in regulating the NK cytotoxic activities in CB vs PB CD56^dim, we compared the genomic expression pattern in CB vs PB CD56^dim.

Methods: For CD56^dim isolation, first, NK cells were isolated indirectly by magnetic separation from non-NK cells. Second, the pre-enriched NK cells (CD56⁺/CD3⁻) from CB and PB were directly labeled with CD16 (FCGR3) MicroBeads, and the CD56⁺ CD16⁺ NK cells (CD56^dim) were eluted after removing the column from the magnetic field (Miltenyi). Purity of CD56dim NK cells were then examined by flow cytometry (BD FACScan). For genomic studies, total RNA was isolated and reverse transcribed to cDNA using T7-Oligo (dT) primer. cRNA was Biotin-labeled by in vitro transcription. Fragmented biotin-labeled cRNA was hybridized to GeneChip U133A_2 in GCOS-operated Fluidics Station 450, and then scanned by GeneChip Scanner 3000 (Affymetrix). Data were analyzed using Agilent GeneSpring. Signal intensities were compared using one way ANOVA and Welch Test for statistical analysis.

Results: There were 193 and 222 genes over and under expressed at the genomic level between CB vs PB CD56^dim NK cells, respectively. CB vs PB CD56^dim significantly overexpressed NKG2A (2.14F), CD16b (2.46F), KIR2D (2.13F), NKp44 (NCR2; 2.62F), PBX1 (4.29F), ENPEP (3.93F). There was no significant difference in NKR gene expression of CD16a, CD161, NKG2C, and NKp46 in CB vs PB CD56^dim. CB vs PB CD56^dim underexpressed the following NK genes: IP3R (1.32F), MAPKAPK5 (1.77F), NCR3 (1.24F), ACACB (3.23F), BBS1 (2.00F).

Conclusion: CB vs PB CD56^dim overexpressed NKG2A, CD16b, KIR2D, and NKp44 genes compared to only NKG2A was overexpressed at the protein level. These results suggest that NKR protein product levels in CB CD56^dim may be directly regulated at the translation level, but not the transcription level. The discrepancy of IP3R, ENPEP, PBX1, and MAPKAPK5 gene expression suggest the involvements of IP₃ and calcium ions in NKR signaling pathways. Since the Notch2, PLEK, and NF-X1 gene expression patterns were not increased, the augmented protein levels may result from the regulation of protein translation. The potential regulators of this process may include PBX1, ENPEP, ACACB, and BBS1 though the roles of these regulators need to be defined. We conclude that genomic differences between CB vs PB CD56^dim may play an important role in regulating NKR signaling pathway, and thus contribute to disparate cytotoxic activity between CB vs PB and suggest a possible explanation for immaturity of cord blood innate and adaptive immunity.