Plasma cells in systemic lupus erythematosus: The longand short of it all

Zheng Liu, YongRui Zou and Anne Davidson

Center for Autoimmunity and Musculoskeletal Diseases, Feinstein Institute for Medical

Research, Manhasset, New York, NY, USA

Plasma cells can be classified as long- or short-lived. The lifespan of a plasma cell largely

depends on whether it arises from a germinal center or an extrafollicular locus and most

importantly whether it can find a survival niche in the spleen or BM. In systemic lupus

erythematosus (SLE) patients, long-lived plasma cells are believed to be responsible for the

production of anti-RNA and anti-cardiolipin antibodies, whereas short-lived plasma cells,

which are more susceptible to anti-proliferation therapies, are the main producers of anti-

DNA antibodies. A previous study showed that transient overexpression of interferon-a

(IFN-a), a cytokine that plays a pathogenic role in SLE, accelerates disease onset in lupus-

prone NZB/W mice. In this issue of the European Journal of Immunology, the same group

report that IFN-a induces large numbers of short-lived plasma cells, accompanied by high

titers of anti-dsDNA antibodies in NZB/W, but not BALB/c, mice. Our commentary

discusses this interesting observation in the context of the previous data regarding plasma

cell differentiation and conveys our view about the clinical implications with respect to

therapies that target plasma cells in SLE patients.

Keywords: Interferon-a . Plasma cells . Systemic lupus erythematosus

See accompanying article by Mathian et al.

Type I IFNs are believed to play a significant role in systemic

lupus erythematosus (SLE) pathogenesis. IFN-a facilitates the

maturation of myeloid DC that contribute to T-cell activation and

follicular T-helper cell differentiation [1], and that produce the

B-cell survival factor B-cell activating factor (BAFF) [2]. IFN-aalso directly stimulates CD4 T cells to enhance antigen-specific

B-cell activation, increases TLR7 expression in B cells, and

promotes T-independent B-cell proliferation and differentiation

into early plasmablasts [3]. In several lupus-prone mouse strains,

type I IFNs accelerate the break in B-cell tolerance to nucleic

acids that occurs spontaneously in these mice with age [4–6].

Nucleic acid-containing immune complexes, in turn, can activate

intracellular TLRs, resulting in further release of type I IFNs and

pro-inflammatory cytokines [7].

Consistent with the known biologic functions of IFN-a,

Mathian et al. [8] show in this issue of the European Journal of

Immunology that NZB/W mice treated with a small dose of IFN-a-

expressing adenovirus develop increased serum levels of BAFF,

IL-6, and TNFa, and high titers of anti-dsDNA antibodies. In

contrast, nonautoimmune BALB/c mice maintain tolerance to

self-antigens despite IFN-a-induced upregulation of inflammatory

mediators. This indicates that a genetic predisposition is required

for IFN-a to initiate autoimmunity and may explain the reason

why only few patients develop SLE during type I IFN therapy [3].

The findings reported by Mathian et al. [8] complement the

recent studies by our group, showing that IgG2a autoantibodies

in IFN-a-induced NZB/W mice were derived from germinal

centers, whereas IgG3 autoantibodies were derived predomi-

nantly from extrafollicular foci. Despite the increased expressionCorrespondence: Dr. Anne Davidsone-mail: adavidson1@nshs.edu

& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu

DOI 10.1002/eji.201041354 Eur. J. Immunol. 2011. 41: 588–591Zheng Liu et al.588

Co

mm

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tary

of TLR7 on B cells, T-cell-independent expansion of marginal

zone B cells and high serum levels of BAFF and IL-6, autoanti-

body production, and clinical disease in this murine model of SLE

is absolutely T-cell dependent [9]. Considering the large number

of germinal centers that form, it is surprising that the multitude of

plasma cells that arise 2–3 wk following IFN-a induction and

persist throughout the disease are short-lived and that they fail to

migrate to the BM or the inflamed kidney or survive for long

periods in the spleen [8, 9].

Differentiation of effector B cells comprises a series of devel-

opmental steps and choices that confer protection against

exogenous threats while minimizing pathogenic autoreactivity

(Fig. 1). The early stages of T-dependent B-cell activation occur

at the B-cell and T-cell border that abuts lymphoid follicles.

Subsequent B-cell fate depends both on the strength of the BCR

signal and on an integrated input of signals from cells and soluble

molecules in the microenvironment. B cells with intrinsically

higher affinity preferentially migrate to the extrafollicular focus

where they rapidly expand and become plasma cells that secrete a

first wave of protective antibodies [10, 11]. Migration to the

extrafollicular focus requires expression of the adhesion molecule

Epstein–Barr virus induced molecule-2 (EBI2) and is enhanced by

strong costimulatory signals, TLR9 ligation, and exposure to

IL-12 [12–15]. This is particularly relevant to SLE because auto-

reactive B cells that recognize and internalize DNA activate TLR9

and may also be exposed to excessive CD40-mediated signals and

cytokines. Although extrafollicular B cells do not undergo

extensive somatic mutation, they may still secrete pathogenic

autoantibodies [12, 13]. Plasma cells derived from the extra-

follicular focus are dependent on BAFF and a proliferation-

inducing ligand (APRIL) for their immediate survival but they are

short-lived, surviving an average of 3–4 days [11, 16].

B cells with relatively low affinity tend to enter germinal

centers; this is mediated through downregulation of EBI2 [14].

Subsequent differentiation into either memory or plasma cells is

regulated in part by the strength of the BCR signal [17]. Plasma

cell differentiation also requires expression of complement

receptor 2 (CR2) [18]. Plasma cells that are fated to migrate to

the BM downregulate CXCR5 (which retains cells in the follicles),

and upregulate S1P1 (which allows egress from peripheral

lymphoid tissues), BCMA (a receptor for the survival factors BAFF

and APRIL), and CXCR4 (the receptor for CXCL12, the major

chemoattractant for circulating plasmablasts) [11].

Whether a plasma cell becomes long-lived largely depends on

whether it finds a microenvironmental survival niche. These

limited and poorly defined survival niches exist in the BM, the

spleen, and the inflamed organs, and involve the interaction of

plasma cells with stromal cells that provide survival factors such

as BAFF, APRIL, and IL-6, adhesion molecules such as VCAM-1

and chemokines such as CXCL12 [11, 19–21]. In murine lupus,

the inflammatory environment, extramedullary hematopoiesis in

lymphoid organs, and lymphoid neogenesis in inflamed target

organs all provide an expanded number of sites where auto-

antibody-producing plasma cells can survive. Despite an increase

in the expression of BAFF and IL-6 and ongoing target organ

inflammation, Mathian et al. observe the failure of plasma cells to

survive long term in NZB/W mice exposed to IFN-a. What can be

the mechanism?

There is some evidence that the failure of plasma cells to

survive in the IFN-a-rich environment is due to their inability to

find survival niches. Liu et al. [9] showed a defect in BM

expression of CXCL12 and VCAM-1 in IFN-a-induced NZB/W

mice. Suppression of CXCL12 in the BM by TNFa, a cytokine that

is upregulated in the IFN-a-induced model, is a feature of acute

inflammation that promotes lymphoid cell mobilization from the

BM [22]. Furthermore, Adalid-Peralta et al. [23] showed both

impaired chemokine expression in the kidneys and a defect in cell

migration that may account for the paucity of renal inflammatory

cells, including plasma cells, in IFN-a-induced NZB/W mice [23].

These findings do not, however, explain the failure of survival of

long-lived plasma cells in the spleens. IFN-a might inhibit the

production of other components of survival niches or might

induce cleavage and deactivation of chemokine receptors. Alter-

natively, plasma cells in IFN-a-induced mice may fail to express

the appropriate receptors or fail to respond to retention or

survival factors in the niches.

In accordance with the observation that plasma cells in the

IFN-a-induced model are short-lived, Mathian et al. further show

that they are susceptible to therapy with cyclophosphamide.

Nevertheless, plasma cells return within several weeks, together

Figure 1. B-cell differentiation to plasma cells in a T-cell-dependentresponse: After T–B-cell interaction, activated B cells with high-affinityBCRs migrate to extrafollicular loci where they differentiate into short-lived plasma cells. This process requires the adhesion molecule EBI2and is enhanced by signals through IL-12, CD40, and TLR9. B cellswith low-affinity receptors downregulate EBI2 expression andenter the germinal center (GC). In the germinal centers, B cells withlower affinity are more likely to differentiate into memory B cells.Upon reactivation, IgG memory B cells are more likely to differentiateinto plasma cells and IgM ones tend to reenter the germinalcenters. Germinal center B cells bearing high-affinity receptorsdifferentiate into plasma cells. This requires downregulation of Pax5,Bcl6, and microphthalmia-associated transcription factor and upregu-lation of IFN regulatory factor-4 and Blimp-1. Some of the plasma cellsdownregulate CXCR5 and upregulate the expression of CXCR4 andsphingosine-1-phosphate 1, allowing them to exit the spleen andmigrate to the BM where they can find survival niches that comprisestromal cells and supporting factors such as BAFF/APRIL, CXCL12, IL-6,TGFb, and VCAM-1. Long-lived plasma cells are believed to express thehighest levels of Blimp-1. IFN-a activates B cells to enter bothextrafollicular foci and germinal centers but inhibits the differentia-tion of long-lived plasma cells. Red arrows, high-affinity BCR; greenarrows, low-affinity BCR; blue font, chemokine receptors and adhesionmolecules; red font, molecules that direct differentiation; green font,BAFF/APRIL receptors; purple font, molecules that are required for BMplasma cell survival.

Eur. J. Immunol. 2011. 41: 588–591 HIGHLIGHTS 589

& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu

with disease relapse as shown by Mathian et al. [8] and also by us

(Liu, manuscript in preparation). The reasons for this remain

speculative. The high levels of BAFF that accompany B-cell

depletion may accentuate the loss of naı̈ve B-cell tolerance to

ubiquitous self-antigens; memory T cells may provide the help

required for the differentiation of these naı̈ve autoreactive B cells

into effector cells. Alternatively, memory B cells could become

reactivated and differentiate into plasma cells. It remains to be

elucidated whether a memory B-cell response is elicited in the

IFN-a-induced model.

How are these findings relevant to human SLE in which type I

IFN plays a pathogenic role? Some autoantibody specificities

(Ro, La, Sm, RNP, and cardiolipin) in lupus patients remain

constant over time, whereas reactivity to dsDNA may fluctuate

with disease activity [24]. SLE flares are often associated with the

emergence of large numbers of plasmablasts in the peripheral

blood [25]. High doses of cyclophosphamide and steroids ablate

circulating plasmablasts and short-lived plasma cells and

decrease titers of anti-dsDNA antibodies in many patients,

whereas the titers of anti-RNA and anti-cardiolipin antibodies are

rarely affected. The plasmablasts are also susceptible to anti-

CD40L treatment, suggesting that they have newly arisen in a

T-dependent fashion [26]. These data in sum have been inter-

preted to mean that anti-dsDNA antibodies are produced by

plasmablasts and short-lived plasma cells, whereas autoantibody

specificities that are not modulated by immune intervention

derive from long-lived plasma cells [24]. It is not clear why, in the

same patient, the plasma cells with different specificities differ in

their lifespan. One possibility is that TLR9 engagement by DNA

preferentially directs DNA-specific B cells to extrafollicular foci

[12]. Alternatively, there might be differences in the signals

downstream of TLR engagement by DNA versus RNA or in the

cytokine microenvironment in which the different specificities

arise. Finally, the non-DNA specificities could give a survival

advantage to plasma cells through an unknown mechanism.

Therapeutic interventions currently used for SLE target

multiple inflammatory cells and deplete plasmablasts and short-

lived plasma cells and therefore deplete a subset of circulating

pathogenic autoantibodies. However, these therapies are asso-

ciated with many unwanted toxicities and with unacceptable

relapse rates. Given that B cells have multiple functions in the

immune system, novel B-cell modulation strategies have been

recently introduced, but these have had only mixed results in SLE

[27]. Rituximab, a monoclonal antibody to CD20, depletes nearly

all peripheral B cells with the exception of plasma cells but does

not prevent flares over the period of a year when used as an add-

on to standard of care therapy in a controlled clinical trial of SLE

patients despite decreasing autoantibody titers [28]. In contrast,

belimumab, an inhibitor of BAFF that depletes naı̈ve B cells with

minimal effects on memory or plasma cells has had modest

therapeutic effects when added to the standard of care therapy in

controlled Phase III clinical trials (R. F. van Vollenhoven, abstract

OP0068 presented at EULAR Congress, Rome 2010 and

S. Navarra, abstract SAT0204 presented at EULAR Congress,

Rome, 2010). These studies show that, even when the source of

plasmablasts and short-lived plasma cells is severely limited by

B-cell depletion, the effect on SLE recurrence is modest at best, at

least in the short term. New reagents such as abatacept and

proteasome inhibitors may be capable of ablating the long-lived

BM compartment. However, plasma cell ablation therapies are

unlikely to prove a mainstay of treatment due to their immuno-

suppressive toxicity and the likelihood of relapse once recon-

stitution occurs. The ongoing challenge in SLE therapy is to find a

strategy that prevents pathogenic autoreactive effector and

memory cells from arising without compromising protective

immunity. This strategy could then be used in early disease, as an

adjunct to ongoing low-level immunosuppression, or after effec-

tor cell ablation therapy.

Murine lupus models are immensely valuable for under-

standing the pathogenesis of SLE and testing novel therapeutic

concepts. However, given their genetically homogenous nature, a

single murine lupus strain can hardly model the full spectrum of

pathogenic mechanisms or immune events that exist in human

SLE. In fact, the longevity and preferred sites of plasma cells are

quite variable among mouse lupus models; this will probably also

be the case in heterogeneous human SLE patients. It therefore

remains to be seen whether the study of Mathian et al. [8] reflects

how IFN-a affects the characteristics of plasma cells that produce

pathogenic antibodies in SLE patients. Nevertheless, this study

illustrates the important concept that even in a homogeneous

genetic background, environmental stimuli such as IFN-a can

change immune events and, therefore, the response to standard

therapeutic intervention. The analysis of patients with and

without an IFN signature or patients before and after receiving

anti-IFN therapy may shed further light on the role of this

cytokine in determining plasma cell fate in human SLE.

Acknowledgements: This work was supported by grants from

the NIH AR 049938-01 and AI 083901.

Conflict of interest: The authors declare no financial or

commercial conflict of interest.

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Abbreviations: APRIL: a proliferation-inducing ligand � BAFF: B-cell

activating factor � EBI2: Epstein–Barr virus induced molecule-2 � SLE:

systemic lupus erythematosus

Full correspondence: Dr. Anne Davidson, Center for Autoimmunity and

Musculoskeletal Diseases, Feinstein Institute for Medical Research, 350

Community Drive, Manhasset, NY 11030, USA

Fax: 11-516-562-2953

e-mail: adavidson1@nshs.edu

See accompanying article:

http://dx.doi.org/10.1002/eji.201040649

Received: 14/12/2010

Accepted: 7/1/2011

Accepted article online: 17/1/2011

Eur. J. Immunol. 2011. 41: 588–591 HIGHLIGHTS 591

& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu