Altered lymphopoiesis and immunodeficiency in miR-142 null mice

Control of gene expression by microRNAs (miRNAs) has recently emerged as a key mechanism that regulates multiple aspects of immune homeostasis, from cell fate determination during hematopoiesis to fine-tuning of immune response to infection.^1,2  Support for this notion was first provided by studies that investigated the consequences of global abrogation of miRNA processing in B and T lymphocytes.

Conditional deletion of Dicer, which encodes an endonuclease required for miRNA biogenesis, in CD19⁺ B cells drastically alters differentiation of peripheral B-cell subsets and results in expansion of marginal zone (MZ) B cells at the expense of follicular (FO) B cells.³  Moreover, germinal center (GC) formation, generation of long-lived plasma cells, and production of high-affinity class-switched antibodies in response to antigen challenge are impaired in mice with specific Dicer ablation in activated B cells.⁴ 

miRNA control is also indispensible for T-lymphocyte homeostasis and function, as T-cell–specific Dicer knockout (KO) mice display smaller peripheral T-cell compartments, as well as defective proliferation and cytokine production by helper T cells.⁵  Although the above-mentioned studies using global ablation of miRNA expression have clearly established an essential role for miRNAs in hematopoiesis and immunity, the challenge today is to determine the contribution of individual miRNA genes to the regulation of immunologic processes and to define the molecular mechanism by which they elicit this control.

miR-142 is one of a few miRNA genes that are preferentially expressed in cells of hematopoietic origin.^6-8  It encodes 2 mature miRNA molecules (miR-142-3p and miR-142-5p) that are derived from opposite strands of a hairpin-like precursor. Both miRNAs are evolutionarily conserved and found in vertebrate genomes from fish to humans.

The biological role of miR-142 remains poorly understood. Knockdown of miR-142-3p expression in zebrafish revealed a critical role for this miRNA in hematopoiesis during embryonic development, hematopoietic stem cell homeostasis, neutrophil differentiation, and heart development.^9-11  In mice, miR-142 is essential for normal development and function of megakaryocytes, dendritic, and mast cells.^12-14 

Our findings from miR-142–deficient mice suggest that this miRNA is necessary for lymphocyte homeostasis, with genetic ablation causing aberrant development of MZ B cells, B1 B cells, and peripheral T cells. Furthermore, we demonstrate that abrogation of miR-142 expression results in a combined immunodeficiency and extend our analysis to implicate miR-142-3p as the functional strand regulating the mature B-cell gene expression program. Finally, we provide genetic evidence that miR-142-3p may act through B-cell–activating factor receptor (BAFF-R) to control B-cell homeostasis. Collectively, our results establish miR-142 as a key regulator of mammalian lymphopoiesis and adaptive immunity, highlighting the importance of posttranscriptional regulation in these processes.

See supplemental Methods (available at the Blood Web site) for additional methods.

miR-142 null mice were generated by deleting ∼900 bp of genomic sequence that encompasses the miR-142 precursor (supplemental Figure 1A). A targeting vector containing 2 LoxP sites, a neomycin-positive selection cassette flanked by FRT sites, and a diphtheria toxin A–negative selection cassette was electroporated into C57BL/6J embryonic stem (ES) cells. The targeted ES cells were microinjected into host C57BL/6J-Tyr^c-2J/J blastocysts and implanted into pseudopregnant foster mothers to produce chimeric mice. The resulting chimeras were bred with C57BL/6J mice to obtain offspring with the targeted miR-142 locus in the germline (F1 generation). F1 mice were crossed with a Cre-deleter strain (C57BL/6Tg(EIIa-Cre)) to remove the miR-142 locus and the neomycin cassette. The resulting miR-142^+/− animals were interbred to produce the miR-142^−/− line. All experimental procedures described in this study used C57BL/6J mice (obtained from The Jackson Laboratory and denoted hereafter as wild-type [WT]) for controls. All animal experiments were approved by the Institutional Animal Care and Use Committee of the City of Hope.

To perform competitive bone marrow (BM) reconstitution assays, 3 × 10⁵ BM cells derived from WT C57BL/6 (CD45.2-positive) or miR-142^−/− (CD45.2-positive) mice were cotransplanted with equal number of WT C57BL/6 (CD45.1-positive) BM cells into lethally irradiated (1000 rad) C57BL/6 (CD45.1) mice. Development of different hematopoietic lineages was analyzed by flow cytometry 12 to 17 weeks posttransplantation.

B cells from 6-week-old WT and miR-142 KO males (n = 3 per genome) were purified from single-cell suspensions of splenocytes using anti-mouse CD19 magnetic beads (Miltenyi Biotec) according to manufacturer’s protocol. Total RNA was isolated using a miRNeasy kit (QIAGEN) and subjected to analysis on a GeneChip Mouse Genome 430A 2.0 Array (Affymetrix). Differential analysis of gene expression in WT and miR-142 KO cells was performed using Partek’s Genomics Suite software. Analysis of enrichment of miR-142-3p and miR-142-5p seed sequences in expression profiles was performed by the Web-based SylArray software algorithm (http://www.ebi.ac.uk/enright-srv/sylarray/). DNA microarray and sample annotation data were deposited in the Gene Expression Omnibus under the accession number GSE61919.

To test experimentally the physiological role of miR-142, we generated a mouse line with a targeted deletion of this miRNA gene (supplemental Figure 1A-B). As predicted, miR-142^−/− mice display virtually no expression of both mature miR-142-3p and miR-142-5p in immune cells (supplemental Figure 1C).

miR-142^−/− mice are born at the expected Mendelian ratios and appear healthy and fertile. Gross morphologic analysis of miR-142^−/− animals at necropsy did not reveal any apparent internal organ defects, except for significant enlargement of the spleen (Figure 1A-B). Splenomegaly was highly penetrant in miR-142^−/− mice but was not accompanied by concurrent lymphadenopathy. Spleen enlargement was characterized by a marked increase in organ cellularity (Figure 1C), suggesting that miR-142 deletion results in an immunoproliferative disorder. Analysis of miR-142^−/− splenocytes by flow cytometry revealed a significant expansion of both B-cell and myeloid cell populations (Figure 1D-G). The number of CD19⁺B220⁺ B lymphocytes in the KO spleen increased ∼2.5 times in comparison to the WT control (Figure 1E).

To probe further the role of miR-142 in B lymphocyte differentiation, we examined the ontogenesis of several B-cell subsets in miR-142^−/− mice. Mammalian B cells are typically divided into 3 main subgroups based on functionality and localization: B1 B cells, MZ B cells, and FO B cells.¹⁵  We observed that the peritoneal cavities of the KO mice were virtually devoid of CD19⁺B220^lo lymphocytes (Figure 2A-B), a cellular population that is typically defined as B1 B cells.¹⁶  Subsequent analysis of miR-142^−/− peritoneal cavity lymphocytes using immunoglobulin M (IgM), CD11b, and CD5 markers further confirmed a marked contraction of the B1 B-cell population (supplemental Figure 2A-B). Deletion of miR-142 diminished production of both CD5⁺ B1a and CD5⁻ B1b B cells, with a greater effect observed in the B1a subset (supplemental Figure 2A-B).

In the spleen, miR-142^−/− B2 B lymphocytes displayed dramatic changes in the expression of CD21 and CD23 surface markers. CD23 expression was downregulated, whereas the level of CD21 increased (Figure 2C). As a result, production of CD21^hiCD23⁻ MZ-like B cells was elevated in miR-142^−/− mice (Figure 2C-D). In contrast, the frequency of the CD21^midCD23⁺ FO B-cell population decreased; however, the total number of FO B cells in miR-142^−/− spleens, although trending lower, was not statistically different (Figure 2C-D and supplemental Figure 5B). MZ B cells normally express high surface levels of the CD1d antigen, which is believed to participate in the presentation of lipid antigens to natural killer T cells.¹⁷  In agreement with such observations, we found a significant increase in the number of B220⁺CD21^hiCD1d^hi cells in the KO spleen (supplemental Figure 2C). Expansion of the MZ-like B-cell population in miR-142^−/− mice was further confirmed by immunohistochemical analysis of spleen sections stained with anti-IgM and anti-mucosal vascular addressin cell adhesion molecule 1 (MAdCam1) antibodies (supplemental Figure 2D). We formally measured the areas of MZ and FO B-cell regions in WT and miR-142^−/− lymphoid follicles. In KO mice, we observed that the average MZ size increased ∼2.4-fold, whereas the average FO zone area contracted (supplemental Figure 2E-F). Consequently, miR-142^−/− spleens displayed an approximately fourfold increase in the ratio between MZ and FO areas (supplemental Figure 2G). The total number of CD19⁺ B cells in miR-142^−/− lymph nodes (LNs) was reduced, although their relative frequency was not altered (supplemental Figure 2J,M). Because mouse LNs are devoid of the MZ structure, these results are consistent with a decrease in FO B cells. In summary, several independent lines of evidence support the notion that deletion of miR-142 results in the expansion of MZ-like B cells. Interestingly, miR-142-3p expression in FO B cells is relatively more abundant in comparison with MZ B cells (Figure 6B), suggesting that lower expression of this miRNA may favor development toward the MZ B-cell lineage.

Our analysis of B-cell maturation in miR-142^−/− spleen revealed a significant increase in both mature (B220⁺CD93⁻) and immature (B220⁺CD93⁺) populations (Figure 2E,G). Further examination of immature B cells with anti-IgM and anti-CD23 antibodies found little change in the frequency of transitional (T1-T3) B cells; however, the absolute number of cells in all 3 transitional stages was significantly increased (Figure 2F,G). We also observed an abnormal accumulation of CD23⁻IgM⁻ B-cell precursors in miR-142^−/− spleens, suggesting that miR-142 deletion perturbs early B-cell development. However, extended analysis of the miR-142 role in early B-cell development is beyond the scope of this study and will be published elsewhere. A higher frequency of immature B cells was also observed in miR-142^−/− LNs (supplemental Figure 2N,O). Taken together, our findings indicate that miR-142 negatively regulates B2 B-cell differentiation and output.

In contrast to B-cell accumulation, our analysis of miR-142^−/− splenocytes revealed a strong reduction in the number of CD3⁺ T cells (Figure 2H-I). Deletion of miR-142 resulted in an approximately threefold drop in splenic T-cell numbers (Figure 2I), but had little effect on the ratio between CD4⁺ and CD8⁺ T cells (supplemental Figure 2H-I). The total number of CD3⁺ T cells was also reduced in miR-142^−/− LNs (supplemental Figure 2J,L). At the same time, thymic development of T lymphocytes appeared normal in miR-142^−/− mice (Figure 2J), suggesting that miR-142 controls either survival or homing of mature T cells to the peripheral lymphoid organs.

Overall, because of the expanding B-cell compartment and declining T-cell numbers, the B- to T-cell ratio in KO spleens was strongly skewed toward B cells (Figure 2K-L). For each T lymphocyte in the KO spleen, we observed ∼5 B cells, whereas the WT mice displayed an almost 1-to-1 ratio (Figure 2L). In agreement with these results, immunohistochemical analysis of the KO spleen sections stained with anti-B220 and anti-CD3 antibodies revealed a sharply diminished T-cell zone and marked enlargement of the B-cell area in lymphoid follicles (Figure 2M and supplemental Figure 2F). Moreover, the B-cell zone in KO spleens lost its normal compact organization and became highly disorganized in shape.

To determine whether defective ontogenesis of lymphocytes in the KO mice is the result of ablation of miR-142 expression in immune cells and not in stromal cells, we performed mixed BM transplantation experiments. Lethally irradiated C57BL/6J mice were injected with miR-142^−/− BM cells together with an equal number of WT BM helper cells (Figure 3A). Gross morphologic examination of resulting chimeric mice 12 to 17 weeks post–BM transplantation revealed moderate splenomegaly in animals that received miR-142^−/− BM (KO/WT→WT), but not in the control group (WT/WT→WT) (Figure 3B). Similar to our observations with miR-142^−/− animals, KO/WT→WT spleens were more cellular than the control organs (supplemental Figure 3). Our analysis of chimeric lymphocyte populations by flow cytometry using 2 isoforms of the CD45 antigen to trace cells of WT (CD45.1/2) and KO (CD45.2) origin revealed several defects. First, we found no CD45.2⁺ B1 B cells in the peritoneal cavities of KO/WT→WT animal cohort, whereas CD45.1⁺ B1 B cells were thriving in the same mice (Figure 3C-D). Second, in KO/WT→WT spleens, the CD45.2⁺ newly formed, immature B-cell population (CD21^intCD23^lo) was ∼4 times bigger than a similar coexisting CD45.1⁺ population (Figure 3E,G). In addition, we found an expansion of MZ-like B cells (B220⁺CD21^hiCD23⁻) in both CD45.1⁺ and CD45.2⁺ compartments in KO/WT→WT, but not WT/WT→WT chimeras (Figure 3E-F). This result implies that miR-142^−/− immune cells can somehow (most likely by secreting a protein factor) drive differentiation of WT B cells toward the MZ lineage. Finally, KO/WT→WT spleens displayed a marked decrease in the number of CD45.2⁺ T cells, whereas the number of CD45.1⁺ CD3⁺ cells did not change significantly (Figure 3H-I). Taken together, our results strongly suggest that aberrant lymphocyte ontogenesis in miR-142^−/− mice was cell autonomous in nature.

The presence of multiple B cell defects in miR-142^−/− mice prompted us to examine the steady-state level of antibody production in these animals. Surprisingly, despite a significant expansion of the B-cell compartment, we found a strong decrease in the levels of all tested serum immunoglobulins in unimmunized miR-142^−/− mice (Figure 4A). In addition, the ability of miR-142^−/− B cells to differentiate in vitro into B220^loCD138⁺ short-lived plasma cells¹⁸  in response to bacterial lipopolysaccharide was greatly diminished (Figure 4B-C). Our findings suggest that the B cells accumulating in miR-142^−/− mice are functionally impaired.

To further explore the consequences of miR-142 deletion on humoral immunity, we immunized mice with a T-cell–dependent (TD) antigen (2,4,6-trinitrophenyl conjugated to keyhole limpet hemocyanin [TNP-KLH]) and analyzed production of antigen-specific antibodies postimmunization. We found that in contrast to a robust immune response to the TNP-KLH challenge in the control mice, the antibody response in the KO mice was largely blunted (Figure 5A). Staining of spleen sections from TNP-KLH immunized mice with peanut agglutinin (PNA) revealed a lack of germinal centers in the KO organs and abnormal accumulation of PNA⁺ myeloid cells in the tissue (Figure 5B). Challenge of miR-142^−/− mice with a T-cell–independent (TI) antigen (TNP-Ficoll) also resulted in a largely unproductive humoral immune response (Figure 5C). Since TI antigens typically do not require T cell help for B cell activation, our findings strongly imply that aberrant antibody production in the miR-142^−/− mice is the result of a B-cell–specific defect. Finally, to examine the role of miR-142 in cellular immunity, we infected the KO mice with herpes simplex virus (HSV-1) and monitored their survival over time. C57BL/6 mice are normally resistant to ocular HSV-1 infection and depend on a robust T cell response to the virus for protection.^19-22  However, miR-142^−/− mice were highly susceptible to infection with HSV-1 and died of virus-induced encephalitis within 11 days (Figure 5D). Lowering the titer of HSV-1 by ∼30-fold did not improve the survival of miR-142^−/− mice. Together, our results suggest that miR-142^−/− mice develop a combined immunodeficiency and as a result are unable to mount productive immune responses to both soluble and viral antigens.

We investigated further how miR-142 deletion can lead to the multiple B cell defects observed in our mutant mice. To this end, we performed expression profiling of purified CD19⁺ WT and KO splenic B cells using DNA microarrays. We found that deletion of miR-142 affects expression of 624 genes (fold change > 1.2 or < −1.2; P < .01 with FDR); 396 genes were upregulated, whereas 228 were repressed in miR-142^−/− B cells. Analysis of differentially expressed genes by the SylArray software algorithm²³  revealed global derepression of miR-142-3p targets, but not miR-142-5p targets, in miR-142^−/− B cells (Figure 6A). SylArray computes enrichment scores for miRNA seed sequences in the 3′ untranslated regions (UTRs) of differentially expressed genes; thus, it can reveal the impact of a specific miRNA gene deletion on the cell transcriptome. Our findings suggest that changes in B-cell physiology in miR-142^−/− mice are most likely the result of abrogation of miR-142-3p expression, whereas miR-142-5p has little contribution to the control of gene expression. This conclusion is in agreement with the significantly lower expression of mature miR-142-5p compared with mature miR-142-3p levels in several B-cell subsets (Figure 6B).

Scrutiny of the list containing the most upregulated transcripts in miR-142^−/− B cells revealed a number of genes with putative miR142-3p binding sites that have well-characterized roles in B-cell development and function (Table 1). In agreement with the notion that miRNAs can fine-tune cellular responses by targeting signaling nodes,²⁴  some of the putative miR-142-3p target genes found to be derepressed in the KO B cells could be arranged into signaling networks. For example, miR-142^−/− B cells display simultaneous upregulation of Wiskott-Aldrich syndrome–like (WASL), cofilin 2 (CFL2), actin filament associated protein 1 (AFAP1), destrin (DSTN), and ras homolog family member Q (RHOQ) genes, all of which function in the actin cytoskeleton remodeling pathway (Table 1). Moreover, recombination activating gene 1 (RAG1), RAG2, and terminal deoxynucleotidyltransferase (DNTT) play key roles in the generation of antigen receptor diversity through VDJ recombination, and all were found among the genes upregulated in miR-142^−/− cells. The third signaling circuit that was also highly represented among the derepressed genes was the BAFF-R pathway, which includes BAFF-R, tumor necrosis factor receptor–associated factor 3 (TRAF3), and p52/p100 nuclear factor κB (NF-κB) subunit (NFKB2).

We employed 3′ UTR luciferase reporter assays to validate BAFF-R, RAG1 and WASL as miR-142-3p direct targets. According to TargetScan^25,26  analysis, the 3′ UTR of mouse BAFF-R gene contains 1 conserved miR-142-3p binding site, whereas the 3′ UTR of mouse WASL has 2 conserved and 2 nonconserved sites (Figure 6C). The single binding site in the mouse RAG1 3′ UTR is not evolutionary conserved (Figure 6C) but can still be bound by miR-142-3p in other species if nonstandard Watson-Crick base pairing is taken into account (supplemental Figure 4). We cloned the 3′ UTRs of WASL, RAG1, and BAFF-R genes downstream of a luciferase reporter and cotransfected them together with a miR-142 expression vector in 293T cells. Expression of all 3 reporter genes significantly decreased in the presence of miR-142, but not after transfection with a control vector (Figure 6D). Introduction of mutations into the predicted binding sites in the 3′ UTRs of BAFF-R, RAG1, and WASL interfered with the ability of miR-142 to downregulate reporter expression, suggesting that miR-142-3p binds directly to these sites (Figure 6D).

Because BAFF-R functions as a direct target of miR-142-3p, we decided to test the notion that dysregulation of this gene contributes to the development of B-cell defects in miR-142^−/− mice. We focused on BAFF-R because of its well-established role in the control of peripheral B-cell growth and survival.^27,28  BAFF-R is a member of the tumor necrosis factor receptor superfamily that exerts its functions by triggering activation of the noncanonical NF-κB signaling pathway²⁹  and the Akt kinase cascade.³⁰  Analysis of BAFF-R expression on the surface of miR-142^−/− B cells by flow cytometry revealed a significant induction of BAFF-R levels (Figure 7A-B). Stimulation of purified WT B cells with BAFF ligand in vitro resulted in processing of the p100/NFKB2 precursor and subsequent accumulation of the p52 NF-κB subunit (Figure 7C). In contrast, miR-142^−/− B cells displayed constitutive activation of the alternative NF-κB pathway (Figure 7C). Moreover, activation of the prosurvival Akt pathway was also altered in miR-142^−/− B cells. Phosphorylated Akt protein was readily detectable in unstimulated cells, and the phosphorylation level was further elevated in response to BAFF ligand. To test how these signaling changes affect miR-142^−/− B-cell functions, we examined proliferative responses of WT and KO B cells. We found that unstimulated miR-142^−/− B cells proliferated at a rate similar to BAFF-activated WT cells, whereas stimulation with BAFF ligand strongly augmented their proliferative capacity (Figure 7D).

Because miR-142^−/− B cells express more BAFF-R, we reasoned that lowering the BAFF-R gene dose would allow us to dissect the functional role of a miR-142-3p-BAFF-R interaction in vivo. To this end, we crossed miR-142^−/− mice with the A/WySnJ strain that carries a naturally occurring B-cell maturation defect-1 (BCMD-1) mutation. BCMD-1 is a loss-of-function mutation that results from a retrotransposon insertion into exon 3 of the BAFF-R gene, encoding the cytoplasmic domain of BAFF-R.^31,32  The resulting miR-142^−/−BAFF-R^+/BCMD1 mice carry only 1 functional BAFF-R allele and show, as expected, a decrease in the expression of BAFF-R mRNA in B cells down to WT levels (Figure 7E). Gross morphologic analysis of miR-142^−/−BAFF-R^+/BCMD1 mice revealed partial rescue of the splenomegaly defect (Figure 7F-G), whereas the total number of splenocytes was significantly reduced (Figure 7H). Moreover, lowering the BAFF-R gene dose also decreased the number of CD19⁺B220⁺ B cells in miR-142^−/− mice and subsequently rescued the B-cell expansion defect (Figure 7I and supplemental Figure 5A). miR-142^−/−BAFF-R^+/BCMD1 spleens displayed normal levels of MZ B cells (Figure 7J), whereas the number of FO B cells dropped slightly in comparison with WT mice (supplemental Figure 5B). We did not observe phenotypic rescue of the B1 B-cell or hypogammaglobulinemia defects in miR-142^−/−BAFF-R^+/BCMD1 mice (Figure 7K and data not shown). Taken together, our findings suggest that miR-142-3p-BAFF-R regulatory axis controls the size of the B2 B-cell compartment, whereas other miR-142-3p targets are probably mediating its effects on humoral immunity and B1 B-cell differentiation.

With a relatively simple mechanism of target recognition, miRNAs have the potential for high evolutionary plasticity, making them ideal regulators of innate and adaptive immunity in organisms continually presented with new pathogenic challenges.³³  The preferential and abundant expression of miR-142 in hematopoietic tissue led us to test the hypothesis that it controls certain aspects of immune homeostasis and function.

Our findings in miR-142^−/− mice implicate this miRNA as a critical regulator of lymphopoiesis. The KO mice display an enlarged splenic B-cell compartment, mostly due to abnormal expansion of MZ-like B cells, as well as contraction of T- and B1 B-cell populations in the periphery. miR-142 plays a cell-intrinsic role in the regulation of lymphocyte differentiation because chimeric mice reconstituted with miR-142^−/− BM develop the same B- and T-cell defects as the miR-142 KO animals. Although the role of miR-142 in lymphocyte development was not a subject of intense research in the past, our observations agree well with prior studies. For example, ectopic expression of miR-142 in BM progenitor cells was shown to hinder B-cell development in vitro.⁷  Furthermore, conditional deletion of Dicer in B cells resulted in preferential expansion of MZ B cells,³  whereas abrogation of Dicer expression in CD4⁺ cells has been shown to reduce the cellularity of the peripheral T-cell compartment.⁵  These results, taken together with the abundance of miR-142 expression in immune cells, make it a prime candidate for a miRNA gene whose deletion is driving the above-mentioned immune defects in the Dicer-deficient mice.

Our results indicate that miR-142 is not only required for normal development of lymphocytes but also indispensable for the optimal function of mature B and T cells. We report herein that miR-142^−/− mice develop a combined immunodeficiency with defects in both humoral and cellular immunity. Challenges of the miR-142^−/− mice with TD and TI antigens failed to yield a robust antibody response. At the same time, miR-142^−/− mice are highly susceptible to infection with HSV-1 virus, an infection model that largely depends on functional CD4⁺ and CD8⁺ T cells for protection.^20,22,34  Our findings suggest that miR-142^−/− mice display a genuine defect in mature B-cell function based on the following observations: (1) failure of the miR-142^−/− mice to mount an effective antibody response to challenge with TI antigen (an immune reaction that arguably depends only on B cells) and (2) defective generation of short-term plasma cells from splenic B cells in vitro. This conclusion is also indirectly supported by the similarity of defects in miR-142^−/− mice and mice with a specific deletion of Dicer in antigen-experienced B cells, which display impaired generation of GCs, plasma cells, and high-affinity antibodies in response to antigen challenges.⁴ 

B-cell defects in miR-142^−/− mice seem to be driven mainly by the loss of miR-142-3p activity, whereas the absence of miR-142-5p expression has little impact. Two pieces of evidence support this notion, including the global derepression of miR-142-3p and not miR-142-5p target genes in miR-142^−/− B cells, as well as the higher abundance of miR-142-3p compared with miR-142-5p in immune cells. Our conclusion is in agreement with observations by others that miR-142-3p and not miR-142-5p is responsible for the regulation of immune cell function in mice^12,13,35  and zebrafish.^9-11 

Growing evidence suggests that miRNAs tightly control cellular responses and organismal homeostasis by targeting signaling nodes in a variety of physiological networks. In agreement with this notion, our study shows that miR-142-3p targets several signaling pathways that play well-established roles in B-cell differentiation and function, including the VDJ recombination complex, actin cytoskeleton rearrangement networks, and BAFF-R signaling cascade. We became particularly interested in the latter pathway because of its proven role in B-cell homeostasis. We provided genetic evidence that the miR-142-3p-BAFF-R axis controls the size of the B2 B-cell compartment, especially the MZ B-cell subset. Loss of miR-142-3p expression elevates the level of BAFF-R on the surface of B cells and results in constitutive activation of the downstream signaling. Consequently, miR-142^−/− mice display dramatic changes in B-cell homeostasis, which to a large degree resemble defects observed in BAFF transgenic mice, including splenomegaly, expansion of the B-cell compartment, and increases in MZ B cells.³⁶  However, not all B-cell phenotypes, such as the B1 B-cell defect and hypogammaglobulinemia, were rescued in miR-142^−/− mice by lowering the BAFF-R gene dose. These results imply separation of function at the level of miR-142-3p targets and, therefore, highlight the relevance and need for future analysis of other targets of this miRNA in B-cell differentiation and function.

Although our work reveals novel insights into miR-142 function, one clinical implication of this study is a rational possibility that mutations in miR-142 gene are behind compromised immunity in patients with immunodeficiencies. Primary immunodeficiency diseases are monogenic disorders that are usually manifested by a wide range of clinical symptoms including persistent infections, autoimmunity, lymphoproliferation, and cancer.³⁷  Intriguingly, miR-142 mutations are found in ∼20% of diffuse large B-cell lymphoma patients.³⁸  Experimental testing of our hypothesis that miR-142 lesions are important etiologic factors for primary immunodeficiency diseases could potentially pave the way for the development of novel diagnostic and pharmacologic strategies to restore immune homeostasis in these patients.

The authors thank Sophia Loera for her help with the processing and analysis of histological samples and members of both the Boldin and Bailis laboratories for helpful discussions and suggestions.

Contribution: N.J.K., W.-L.W., E.Y.R., and M.P.B. designed the study, developed methodology, collected and analyzed data, and wrote the manuscript; B.K. and C.-C.C. assisted with collection and analysis of data from mixed BM chimeras; C.R. and E.M.C. performed and analyzed data from HSV-1 infection experiments; K.D.T. and N.C. assisted with collection and analysis of expression profiling data; and S.L.V. performed histopathological analysis of tissue sections.

Conflict-of-interest disclosure: N.C. is a shareholder of Regulus Therapeutics, a biotech company developing miRNA-based drugs. The remaining authors declare no competing financial interests.

The current affiliation for K.D.T. is EMD Millipore, Temecula, CA.

Correspondence: Mark P. Boldin, Department of Molecular and Cellular Biology, Beckman Research Institute, City of Hope, 1500 East Duarte Rd, Duarte, CA 91010-3000; e-mail: mboldin@coh.org.