Inducible nitric oxide synthase is crucial for plasma cell survival

Abstract

The viability of long-lived plasma cells is enhanced by the expression of inducible nitric oxide synthase, which relieves endoplasmic reticulum stress by triggering a response dependent on cGMP and protein kinase G.

Plasma cells, which are terminally differentiated B cells, are the workhorses of humoral immunity. There are two types of these cells: short-lived plasma cells and long-lived plasma cells¹. Short-lived plasma cells are generated within a few days after infection and die after 2 weeks. The antibodies secreted by short-lived plasma cells help to control the infection. Meanwhile, long-lived plasma cells, which are generated through the germinal-center pathway, migrate and take up residence in the bone marrow and continue to secrete copious amounts of antibodies, usually throughout the host’s lifespan. Long-lived plasma cells confer long-term protection to the host and hence their longevity is pivotal in their ability to provide long-lasting immunological protection. Long-lived plasma cells reside in the bone marrow, where stromal cells, eosinophils and basophils provide survival factors, such as interleukin 6 (IL-6), the chemokine CXCL12 and the proliferation-inducing ligand APRIL, that facilitate their survival¹. In this issue of Nature Immunology, Anna George and colleagues demonstrate that inducible nitric oxide synthase (iNOS) is an intermediate in signaling pathways that promote the survival of plasma cells². They show that deficiency in iNOS results in a shorter lifespan for plasma cells, while it has no effect on the activation and terminal differentiation of B cells. Through studies of iNOS-deficient plasma cells and iNOS inhibitors, they demonstrate that iNOS is involved in signaling via IL-6 and APRIL, both of which are critical for the survival of plasma cells in the bone marrow^3,4. Furthermore, they show that iNOS probably promotes the survival of plasma cells through a pathway of iNOS, nitric oxide (NO), cGMP and protein kinase G (Fig. 1).

Figure 1.

A working model for iNOS-mediated enhancement of plasma cell survival. During the terminal differentiation of plasma cells and the UPR, XBP-1 is cleaved, and it induces iNOS expression. Then, iNOS catalyzes NO production, which in turn promotes the survival of plasma cells by serving as an intermediate in signaling via APRIL and IL-6. At present it is unknown where NO intersects with the signaling cascades of APRIL and IL-6. IL-6R, receptor for IL-6; BCMA, B cell–maturation antigen; TACI, transmembrane activator; PC, plasma cell; Jak, kinase; STAT, transcription factor; MAPK, mitogen-activated protein kinase; NF-κB, transcription factor.

NO is the smallest known signaling messenger; it has numerous pivotal functions in mammalian physiology. It is produced by three isoforms of nitric oxide synthases: endothelial nitric oxide synthase, neuronal nitric oxide synthase and iNOS⁵. All these are homodimers that catalyze the production of nitric oxide from L-arginine. While the endothelial and neuronal nitric oxide synthases are constitutively expressed and initiate a wide range of cellular processes, including myocardial function and neurotransmission, iNOS is transcriptionally regulated and is induced by cytokines, lipopolysaccharide and other microbial agents⁵. As indicated by its mode of activation, iNOS serves as a signaling mediator during host responses to infection and it is also associated with septic shock. The role of iNOS in macrophages has been extensively studied; when induced, macrophages produce a large amount of NO, which facilitates the elimination of microbes⁵. Published studies have shown that iNOS can be induced in other cells of the immune system and those not of the immune system and can mediate the elimination of microbes, parasites and tumor cells⁵. Interestingly, iNOS is also associated with both cell death^6,7 and cell survival^8,9. Prior to this insightful study by George and colleagues², there was no direct link between iNOS and the survival of plasma cells. However, a published study demonstrating that iNOS modulates endoplasmic reticulum (ER) stress¹⁰ hinted a possible role of iNOS in the function of plasma cells.

The ER stress response occurs when excess proteins accumulate in the ER. This response occurs in plasma cells, since they continuously synthesize large quantities of antibodies. To cope with that intense protein production, plasma cells induce the unfolded protein response (UPR), which increases the efficiency of protein processing and prevents the apoptosis that would otherwise ensue with unrestrained ER stress. There are three main molecular branches of the UPR: the serine-threonine kinase and transmembrane endonuclease IRE-1, the ER stress–resistant kinase PERK, and the transcription activator ATF-6. Activation of those molecules results in the production of the transcription factors XBP-1, ATF-4 and ATF-6(N), respectively; these activate genes encoding molecules that facilitate protein processing in the ER¹¹. The activation of PERK and IRE-1 leads to the induction of genes encoding molecules that promote mRNA decay, to decrease the protein-folding load of the ER. In contrast, ATF-6 activates genes encoding molecules that increase protein- folding capacity of the ER. XBP-1 is a transcription factor and signaling molecule that is activated by and signals downstream of IRE-1 and ATF-6 in the UPR. It exists in two forms: unspliced and spliced. ER stress activates IRE-1, which processes XBP-1 mRNA to generate the spliced version of the protein, which activates genes encoding enzymes that degrade misfolded proteins. Spliced XBP-1 also promotes expression of the gene encoding NOS; interestingly, the ensuing increase in NO also modulates ER stress by resulting in the upregulation of genes encoding ER-resident chaperones¹². Consistent with those reports, the present study by George and colleagues suggests that iNOS expression in plasma cells promotes protein processing during the ER stress response². In the absence of iNOS, they detect a lower abundance of mRNA encoding XBP-1, Gadd34, EDEM1 and GRP94, all of which are involved in alleviating ER stress. However, it is yet to be determined whether NO has an effect on the abundance of spliced XBP-1, which mediates ER stress relief. The results of published reports¹⁰ and the present study² would indicate that iNOS and XBP-1 might regulate each other; however, further studies are needed to confirm this hypothesis.

George and colleagues have brought awareness of the involvement of iNOS and NO in the lifespan of plasma cells²; this contribution will have tangible effects on the future of research into plasma cells. For example, it may be worth exploring whether iNOS-NO can be used in attempts to enhance the longevity of plasma cells, especially in a vaccine setting. The physiological and potential pathological roles of iNOS and NO in plasma cells should also be scrutinized, given that in macrophages, the role of iNOS-NO has been studied extensively, and it is known that the NO induced is sometimes toxic to healthy cells, which leads to tissue damage and possibly nonspecific allograft rejection. Finally, it would be of great interest to determine whether iNOS and NO operate in myelomas and if their down-modulation could be used as a therapeutic tool in the fight against plasma cell neoplasias. Likewise, the role of iNOS-NO could also be explored in the context of modulating plasma cells in autoimmunity.