CD19 is a 95 kDa transmembrane glycoprotein of the immunoglobulin superfamily encoded by the CD19 gene that is expressed under the control of the transcription factor BSAP (encoded by PAX5) 59 60 61 , a master regulator of B-cell identity 62 63 . No other transcriptional regulator has been identified to bind to the CD19 promoter 59 , so CD19 expression probably recapitulates the activity of PAX5/BSAP. Consistently, CD19 is expressed by B lymphocytes but not by any other cell type, except for an unconfirmed report suggesting CD19 expression by some follicular dendritic cells 64 .

In B cells, expression of CD19 (also referred to as B4 antigen) starts prior to the expression of CD20 (B1 antigen) at the late pro-B-cell stage together with that of CD10 65 66 67 . CD19 expression is maintained throughout various B-cell differentiation and activation stages until PC differentiation, which is then associated with partial loss of CD19 expression (Figure 1).

Regular expression of CD19 appears to require CD81, as CD81-deficient mice show a 50% decrease in CD19 expression as compared with controls 68 and a patient deficient in CD81 did not express CD19 on the B-cell surface despite the expression of intact CD19 alleles 69 . BCR deficiency and mutations in Btk are associated with higher and lower CD19 expression by circulating human B cells, respectively 70 . Mutations within one allele of CD19 itself lead to ~2.4-fold reduced surface expression of CD19 71 , while homozygous mutations in the exons encoding cytoplasmic residues of CD19 result in the generation of a truncated, dysfunctional CD19 molecule that is not recognized or is poorly recognized by standard anti-CD19 antibodies 72 . In almost all cases, dysregulated CD19 expression is associated with abnormalities of the immune system, as also suggested by the development of autoantibodies in mice carrying a human CD19 transgene leading to overexpression of CD19 73 . Consistently, CD19 expression has also been reported to be dysregulated in peripheral B cells from autoimmune patients.

Overall, CD19 is expressed by almost all developmental and activation-dependent B-cell differentiation stages, except for certain PCs, and functions as an important regulator of specific B-cell activation by antigen (Figure 1). CD19 expression coverage exceeds that shown by CD20 expression. Whether all types of B cells will be similarly targeted by therapeutic anti-CD19 antibodies requires first experiences in patients treated with such antibodies.

On the surface of B cells, CD19 functions as a co-receptor of the BCR and contains two extracellular C2-type Ig-like domains. Through these domains, CD19 interacts with CD21 (CR2, C3d fragment receptor) and CD81, forming together with CD225 the BCR complex 74 .

The intracellular domain of CD19 is involved in intracellular signaling cascades, primarily but not exclusively regulating signals downstream of the BCR and CD22 58 . Mice either lacking endogenous CD19 expression or overexpressing a human CD19 transgene under control of the endogenous promoter revealed that CD19 is essential for the normal function of the BCR, and in particular regulates the threshold for its activation. While CD19-deficient mice show impaired B-cell responses, B cells from hCD19 transgenic mice were hyperresponsive 75 76 and produce autoantibodies. CD19 is thus a regulator of BCR signaling threshold and is implicated in the control of peripheral tolerance 77 . Human CD19 deficiency has no apparent impact on B-cell development but relates to the phenotype of CD19^-/- mice, in that these patients suffer from antibody deficiency 72 . CD19 has also been suggested to have a role in regulating cell proliferation, apparently independent of B-cell identity. CD19 transfection of multiple myeloma but also other cell lines led to reduced culture growth and tumor formation when injected in mice. This effect depended on the cytoplasmic part of CD19 78 . Overall, the currently available data suggest that CD19 plays critical roles in B-cell and PC differentiation as well as in cell turnover.

In the blood of patients with lupus, ANCA-associated vasculitis and systemic sclerosis, but also in healthy individuals, CD27⁺ (memory) B cells appear to express higher levels of CD19 than CD27^- (naïve) B cells 79 80 81 , perhaps corresponding to the distinct activation threshold of memory versus naïve B cells in response to antigen. Moreover, circulating CD27^high-expressing plasmablasts usually but not always show downregulated, however clearly detectable, expression of CD19 in autoimmune patients and healthy individuals 80 81 82 83 84 . The cell surface density of CD19 is lower than that of CD20 in normal B cells 85 . This difference probably has limited implications for the functionality and biological efficacy of anti-CD19 versus anti-CD20 treatments, because specific modifications of the antibodies (engineering) preferentially define target cell binding and clearing properties 86 87 .

In autoimmune patients, both reduced and enhanced CD19 expression by peripheral blood B cells has been described compared with healthy controls. Patients with systemic sclerosis displayed a ~20% increase of CD19 expression on peripheral blood B cells irrespective of whether naïve or memory B cells were analyzed 73 81 . CD19 expression levels correlated with serum IgG and IgM concentrations but not with serum anti-nuclear antibody in these patients 73 .

In our analyses, average CD19 expression levels of memory and naive B cells from patients with SLE were comparable with those of healthy controls based on mean fluorescence intensities, whereas plasmablasts showed a trend towards lower CD19 levels (Figure 2). However, CD19 levels detected on patients' memory cells and also naive B cells showed higher variability than those of healthy controls. Stratification of the patient cohort according to high and low disease activity (Systemic Lupus Erythematosus Disease Activity Index (SLEDAI)) or ascending expression levels of CD19 suggested an inverse correlation of CD19 expression levels with disease activity. Moreover, CD19 levels expressed by plasmablasts, memory cells and naive B cells strongly correlated with each other (P < 0.0001). Overall, CD19 expression by plasmablasts was significantly lower as compared with that of memory cells and naive B cells in both SLE patients and controls. This suggests that CD19 expression levels in B cells are regulated via differentiation-independent mechanisms as well as differentiation-dependent mechanisms. Other studies documented about 20% reduced CD19 expression levels by naïve and memory B cells from SLE patients 73 81 . In this regard, our SLE cohort appears to comprise patients with either markedly higher or lower CD19 expression, while there were only few patients within the limited range of CD19 expression values of healthy individuals. These data reproduce both upregulation and downregulation of CD19 by blood B cells from autoimmune patients, which may correspond to the previous observations of CD19^high B cells (as discussed below) or reduced CD19 expression 81 .

Overall, the reduction of CD19 expression by SLE B cells in at least some patients appears to be a reproducible phenomenon, independent of disease activity, the presence of anti-nuclear antibodies or use of the different anti-CD19 antibodies applied to expression analyses 79 80 . Interestingly, the difference of CD19 expression by B cells between healthy controls and SLE patients has not been found on a transcriptional level and was reversible after a 2-day, nonstimulating in vitro culture 80 .

These data representing an apparent bidirectional dysregulation of CD19 expression by B cells from autoimmune patients are consistent with the fine-tuning of CD19 expression in vivo, which however may not be specifically linked to autoimmunity in general.

Additional studies have characterized a population of CD19^high-expressing B cells in patients with SLE and ANCA-associated vasculitis, representing up to 10% of the B cells in most patients and up to 30% in individual cases 79 88 89 . CD19^high B cells showed markedly higher CD19 expression than B cells from healthy controls, while the remaining CD19^low B cells recapitulated the reduced CD19 expression 79 as also documented by other studies 73 80 81 . Detailed flow cytometric analyses of CD19^high B cells revealed increased forward-scatter signals compared with CD19^low B cells. CD19^high B cells mainly expressed CD27, and to a lesser degree IgD, and displayed increased expression of CD86, MHC II and CD95 79 89 consistent with recent activation of this putative memory B-cell subset in vivo. Consistently, CD19^high B cells displayed enhanced phosphorylation of signaling molecules downstream of the BCR complex, such as Syk and ERK, which distinguished them from CD19^low B cells 88 and could be generated in vitro from both BCR/CpG-activated naive or memory B cells 90 .

In SLE and other autoimmune and immune-deficient patients, CD19^high B cells are characterized by low surface CD21 expression 70 73 79 81 89 . These cells therefore resemble the phenotype of B cells that were reported as 'exhausted tissue-like memory B cells' in patients with HIV 91 or common variable immunodeficiency 92 . In the latter, CD19^high B cells were not associated with autoimmune phenomena but with splenomegaly and cytopenia 92 . The presence of CD19^high/CD21^low B cells is hence not unique for patients with SLE and probably corresponds to distinct functional stages of activated B cells. In support of this hypothesis, such CD19^high B cells express reduced levels of CD62L (L-selectin), CXCR4 and CXCR5 70 89 91 , which may exclude these cells from regular recirculation and germinal center participation, while enhanced expression of functional CXCR3 88 might allow for their migration to sites of inflammation in vivo.

The presence of CD19^high B cells did not correlate with disease activity in SLE or ANCA-associated vasculitis 79 89 , but did correlate with neurological and renal disease manifestations 79 88 , shorter clinical benefit from rituximab treatment 88 as well as anti-Sm but not anti-nuclear antibody or anti-Ro/La titers 79 . Indeed, CD19^high B cells from anti-Sm-positive SLE patients yielded considerable amounts of Sm-specific antibody-secreting cells 6 days after in vitro activation culture, in contrast to CD19^low B cells 88 , suggesting that this subset might contain a substantial part of autoreactive B cells.

In summary, abnormalities in CD19 expression appear to be related to the activation status of B cells rather than specifically linked to autoimmunity (Table 3). The combined data of CD19 expression in conjunction with changes of CD21, CD81, BCR and Btk suggest that CD19 expression is fine-tuned by BCR-mediated activation signals. Given the enhanced CD19 expression by some B cells under the condition of autoimmunity, these cells may constitute a key target. However, given that blood lymphocytes represent approximately 2% of total body lymphocytes 93 94 , whether the data above are also representative for tissue-resident B cells in the spleen, lymph nodes, BM, gut-associated lymphoid tissues and inflamed sites remains open. Apparently all B-cell stages in the human blood and probably in the relevant tissues express CD19 and presumably could be targeted by an anti-CD19 antibody.

Importantly, a substantial fraction of antibody-secreting PCs express CD19 in contrast to CD20, which is downregulated during plasmablast differentiation in association with their relocation from secondary lymphoid tissue to PC depots in the BM or lamina propria. Splenic and tonsillar plasmablasts appear as CD19⁺CD20^{+/intermediate}- expressing cells 95 96 97 , while plasmablasts in the blood express low or no CD20 and diminished CD19 levels 80 81 83 84 96 98 . One must emphasize that PCs in the BM consist of both CD20^-CD19⁺ and CD20^-CD19^- PC subsets in man 96 99 and in the mouse 100 . The bimodal distribution of CD19 expression among PCs in the BM of both healthy and autoimmune individuals (Figure 3) has important implications since CD19-directed therapies could therefore target some, but not all, PCs.

Both PC differentiation and downregulation of CD19 are tightly interrelated with the loss of PAX5/BSAP expression 101 102 , and consequently BSAP binding sites in the CD19 promoter are not occupied in PCs 59 . While many PC lines and multiple myeloma cells lack CD19 expression, fractions of normal mature PCs in the human BM but also in the mucosal lamina propria express CD19 96 99 103 104 105 , including IgG-secreting cells in man 98 and Blimp-1⁺ PC in the mouse 100 .

Little is known about the nature of underlying immune reactions and/or the relationship between CD19⁺ and CD19^- PC in the BM or lamina propria, which makes it challenging to estimate the potential effects of CD19-directed therapy on humoral (auto)immunity. In the mouse, the gradual loss of CD19 on PCs seems to be associated with advanced maturity; that is, higher expression of Blimp-1, lower expression of HLA-DR and B220, and eventually a longer lifespan 100 . Neither CD19⁺ nor CD19^- PCs from human BM expressed PAX5 mRNA, but they did express IRF4, PRDM1 and CD138 at comparable levels (n ≥4; HE Mei and T Dörner, un published data). This suggests additional factors regulating the expression of CD19 on PCs beyond transcriptional control by PAX5/BSAP.

These data are consistent with the assumption that anti-CD19 therapy would probably target significant fractions of PCs while not enhancing all PCs. More information is therefore required on the dynamics of and the relationship between CD19⁺ and CD19^- PCs in order to estimate the impact of anti-CD19 treatment.

CD19⁺ cells comprise virtually all antibody-secreting cells in the peripheral blood in both healthy individuals and patients with SLE as well as other autoimmune diseases 25 83 106 . These comprise (at least) two major fractions: first, recently generated antibody-secreting cells that undergo cell division, are capable of in vitro migration towards gradients of CXCL12 25 , express high levels of HLA-DR and contain antigen-specific cells that pass the peripheral blood 1 week after a tetanus booster vaccination 83 . We termed these cells plasmablasts 7 25 36 83 , whereas the remaining antibody-secreting cells were defined as PCs that were unable to proliferate and migrate in vitro 25 , lack surface expression of the adhesion molecules β₇-integrin and CD62L (L-selectin), express low levels of HLA-DR and do not contain antigen-specific cells after vaccination 25 83 . When analyzed 1 week after tetanus vaccination, circulating HLA-DR^low PCs phenotypically, functionally and transcriptionally showed a high degree of similarity with BM PCs 7 25 , and probably reflect outcompeted mature BM PCs 83 . Both subsets can be detected by flow cytometry after gating on CD19⁺ cells combined with an enlarged lymphocyte gate 25 82 83 84 .

CD19 expression by plasmablasts in autoimmune diseases has rarely been analyzed. Sato and colleagues did not detect statistically significant differences in the CD19 signals by flow cytometry when comparing plasmablasts (together with PCs using the CD27^high criterion for detection) among healthy individuals and patients with SLE or systemic sclerosis 81 - while another study showed lower CD19 expression by SLE CD27^high plasmablasts as compared with those from healthy controls 80 , which is consistent with our data (Figure 2). As illustrated in a representative SLE patient, approximately 80% of the peripheral blood CD138⁺ plasmablasts/PCs express CD19 84 , indicating that a minority of circulating PCs lacks expression of this marker.

Frequencies and numbers of CD19-expressing plasmablasts/PCs are drastically increased in patients with SLE 54 82 84 107 and reflect up to 50% of all circulating CD19⁺ B cells 84 while containing dsDNA-specific cells 54 . The cell frequency and number correlated with disease activity and autoantibody production 82 84 106 , and this correlation was even more pronounced when HLA-DR^high plasmablasts were analyzed separately 54 . The presence of these plasmablasts therefore reflects acute, presumably pathogenic B-cell activation in patients with SLE. Elevation of peripheral blood plasmablast levels has also been recently demonstrated for Takayasu's arteriitis patients 108 , but not in patients with primary Sjögren's syndrome 109 , autoimmune thrombocytopenia 110 , systemic sclerosis 81 and RA 19 . Further analyses focusing on CD19⁺ B cells in the BM demonstrated an imbalance of HLA-DR^high plasmablasts versus HLA-DR^low PC subsets - that is, relative expansion of plasmablasts in SLE patients' BM - indicating that not only the periphery but also the major deposit sites of PC are disturbed in SLE 54 .

The prominent presence of presumably pathogenic plasmablasts in SLE patients' blood and BM 3 provide a further rationale for anti-CD19 therapy since CD19⁺CD20^-/low plasmablasts are refractory to B-cell depletion with rituximab in RA, SLE and other autoimmune syndromes 19 21 111 112 113 114 . In this context, we found that rituximab-resistant plasmablasts carry a mucosal phenotype in patients with RA and show multiple traits of their recent generation, such as cell proliferation and spontaneous in vitro migration. This finding points towards a self-sufficient, rituximab-resistant mucosal B-cell subset in man 19 that is chronically activated and gives rise to plasmablasts in circulation. In SLE patients, higher levels of plasmablasts after B-cell depletion have been reported to be associated with poorer clinical response 112 . Anti-CD19 therapy might constitute a possibility to overcome this limitation of rituximab by directly targeting these plasmablasts and certain CD19⁺ PCs. There is thus a possibility that especially autoantibody-positive patients could benefit from anti-CD19 therapy, but this needs to be demonstrated.

Overall, a better understanding of the homeostasis and differentiation of different mature PC subsets in healthy subjects and autoimmune patients not only related to the expression of CD19 but also to their distribution and characteristics in certain organs and organ systems is required to fully explore the potential benefits and potential side effects of, and suitable molecular targets for PC-directed therapies.

Initial studies in non-Hodgkin's lymphoma patients 115 116 using CLB-CD19 with or without IL-2 demonstrated binding of anti-CD19 antibody to B cells in vivo and led to partial reduction of blood B cells, but did not consistently change total antibody levels during treatment (up to 12 weeks) and in the follow-up period.

Various anti-CD19 immunotoxins are under development for malignant diseases - for example, bound to auristatin 117 , saporin 118 or ricin 119 - but have not so far entered studies in autoimmunity.

In a mouse model of anti-CD19 therapy using hCD19 transgenic mice 120 and anti-human CD19 antibodies (HB12a, HB12b and FMC63), depletion of >90% of B cells in the BM, blood, spleen and lymph nodes was observed, while peritoneal cavity B cells were reduced by only two-thirds. Notably, pro-B cells in the BM were not depleted in this system. Blood B cells remained depleted for about 12 weeks and only partially repopulated until 30 weeks after treatment. In this model, levels of total serum antibody concentrations were reduced by 95% for IgM, by 10 to 20% for IgG and by about 40% for IgA compared with baseline values. Specific IgM and IgG1 responses to T-independent antigens and primary and secondary responses to T-dependent antigens were virtually abrogated in mice treated with anti-CD19. Reduction of specific serum IgM and IgG₁ concentrations also occurred in previously immunized mice treated with anti-CD19 120 , most probably reflecting that antibody-secreting cells were successfully targeted in this model.

Autoantibody production analyzed in this mouse model was critically dependent on the overexpression of CD19 121 122 , and CD19 expression was reduced over at least 10 weeks after anti-CD19 therapy. While animals treated with control antibody developed increasing levels of anti-dsDNA, anti-ssDNA and anti-histone IgM and IgG antibodies, anti-CD19-treated mice showed reductions of IgM autoantibodies within 2 weeks. IgG autoantibodies were also reduced in a delayed manner at the latest by week 10 after treatment 120 . These Ig-class-specific responses of serum antibody levels to anti-CD19 treatment indicate differential dynamics of IgM⁺ and IgG⁺ PC subsets and indicate that some but perhaps not all autoreactive PCs express CD19 in this model. However, additional studies are required in control mice and in models of SLE that are independent of genetic modification of CD19 expression.

Modifications of the amino acid sequence and the glycosylation of native anti-CD19 antibodies 86 87 , as well as engineering of bispecific antibodies co-targeting other antigens (for example, CD3 123 ) to enhance target cell binding clearing properties, have generated interest in anti-CD19 immunotherapy.

In this context, the bispecific CD19×CD3 antibody (Blinatumomab, MT103, MEDI-538, bispecific T-cell engager (BiTE)) that has been designed to recruit and activate T cells in the vicinity of CD19⁺ tumor cells in non-Hodgkin's lymphoma patients resulted in substantial and efficacious depletion of blood B cells (residual numbers <10 cells/μl) for up to 2 months and obviously cleared malignant B cells in the BM, spleen and liver. More than 85% of the study participants showed no significant decline of serum Ig levels, reflecting that most patients retained an intact PC compartment during therapy.

The application of XmAb5574 (MOR208) in Cynomolgus monkeys did not reduce serum IgG, IgA and IgM levels over a 3-month period 124 , indicating that the PC compartment remained intact either due to resistance of PCs or incomplete depletion of B cells in this model. Indeed, only 80 to 90% of CD20⁺ B cells in peripheral blood and 40 to 60% within the BM, spleen and lymph nodes were depleted 1 month after anti-CD19 treatment 124 . A clinical phase I trial with XmAb5574 in chronic lymphocytic leukemia patients is currently ongoing (Trial #NCT01161511). A further anti-CD19 antibody is MEDI-551, which is able to deplete B cells in suspensions of human peripheral blood mononuclear cells as well as blood and splenic B cells in hCD19/hCD20 double transgenic mice 86 . Data that might allow an evaluation of targeting PC in humans by this antibody are not yet available, but clinical trials have been initiated. The anti-CD19 antibody MDX-1342 was under investigation in RA patients in combination with methotrexate 125 but was discontinued in April 2010. Further anti-CD19 approaches such as derivates of the anti-human CD19 clone HD37 are being developed 126 .

Multiple anti-CD19 antibodies are being and have been developed for therapeutic use, so far with very inconsistent experiences in preclinical models or pilot trials with regard to both the efficacy and duration of B-cell and PC depletion.

A different way of CD19 targeting employs transplants of autologous T lymphocytes that are isolated from the patients' blood, stimulated in vitro with, for example, anti-CD3 and IL-2, and transduced with a vector conferring expression of a chimeric antigen receptor (CAR) that is directed against CD19. Upon incubation with CD19⁺ cells, CAR-bearing T cells produce IFNγ and IL-2 127 128 and lead to sustained depletion of B cells, including tumor cells in human blood (at least for 40 weeks) and in BM (at least for 36 weeks) 127 . Notably, serum Ig levels gradually dropped during this therapy with reductions of approximately 40 to 50% for IgG within 14 weeks, 85% (within 14 weeks) and 75% (until week 36) for IgA and IgM, respectively. In a subsequent study of eight non-Hodgkin's lymphoma patients, four individuals experienced profound B-cell depletion with hypogammaglobulinemia requiring Ig substitution 129 - similar to that described for treatment of chronic lymphocytic leukemia patients using CD19 CAR-expressing T cells 130 . Depletion failed in the other four patients, as has also been reported by an independent study 131 , and was probably due to properties of the malignant B cells or result from variations of CD19 chimeric antigen receptor T-cell generation.

The available data on B-cell and PC targeting by anti-CD19-directed approaches in oncologic diseases do not allow firm conclusions as to what extent PC depletion occurs in autoimmunity and may differ between certain agents and techniques. Ongoing clinical studies will provide more detailed information on the efficacy and especially safety of anti-CD19 approaches, which will also permit a unique opportunity to compare those with anti-CD20 experiences and at the same time better define potential treatment options for autoimmunity.

Apparently, targeting both B cells and PCs raises safety concerns with regard to sufficient control of environmental pathogens and infections by protective Ig generated as a first-line defense as well as during B-cell memory responses. CD19 targeting can be assumed to enlarge the window of cell depletion compared with anti-CD20 therapy, with an additional impact on pro-B cells and PCs. As the maintenance of protective and autoreactive antibody titers and the contributions of different subsets of CD19⁺ versus CD19^- PCs and CD19⁺ plasmablasts are incompletely understood, one may only speculate about the impact of anti-CD19 on immune protection. Successful targeting of CD19⁺ PCs in patients with SLE is expected to at least substantially reduce autoantibody titers produced by plasmablasts and certain PCs. However, severe reductions of Ig levels can be managed by intravenous Ig infusions 129 . We believe one cannot exclude the possibility that patients might experience loss of or impairment of sufficient antibody production under anti-CD19 therapy, as observed in patients with common variable immunodeficiency who often show reduced numbers of PCs in the BM 132 and usually fail to mount sufficient vaccination responses 133 . The failure to sufficiently respond to vaccinations of most patients treated with anti-CD20 will probably also affect patients treated with anti-CD19 therapy. During rituximab treatment, both primary and secondary serological antibody responses are substantially impaired 134 135 136 137 138 , while pre-existing antibody titers were largely maintained. The estimated time period of B-cell absence after anti-CD19-mediated B-cell depletion is currently unknown, while B-cell depletion with rituximab lasts usually 6 months or longer. Given the broader coverage of anti-CD19 that includes pro-B cells, it is possible that peripheral B-cell repopulation after anti-CD19 therapy differs from that after anti-CD20 therapy.

A situation of antibody deficiency has been described in SLE patients after ASCT with prior immunoablation using high-dose cyclophosphamide and rabbit antithymocyte globulin, which often leads to improvement of the disease, perhaps remission, but also caused loss of protective serum antibody titers directed to both protein and viral antigens (anti-tetanus, anti-measles, anti-diphtheria, anti-rubella) 5 . The risk of opportunistic infections therefore requires careful monitoring and potentially might represent a limitation. Given that diminishing autoimmune PCs provides substantial benefit for the patients, including long-term remission, the possibility of intravenous Ig substitution may allow balancing the risk of hypoimmunoglobulinemia and infections.

Experiences with this therapy in tumor patients 123 127 129 131 might in part be instructive for potential risks of anti-CD19 therapy in autoimmunity but can only be extrapolated with caution because of the different underlying disease and pre-existing and co-medications. Further potential side effects of anti-CD19 therapy probably overlap with various adverse events reported after B-cell depletion with rituximab, such as the occurrence of infections 139 140 , suppression of specific B-cell responses 134 135 136 and risk to develop progressive multifocal leukoencephalopathy 141 . These aspects are of particular importance since B-cell depletion by anti-CD19 strategies appears to be broad and long-lasting 129 while B cells and antibody is required to suppress John Cunningham virus (see 141 ). Overall, the safety profile of anti-CD19 therapy in autoimmunity will be of central importance but requires further data.

Animal model data for anti-CD19 therapy documented efficient reductions but sometimes not complete depletion of B cells 120 124 . In such cases, B cells labeled by anti-CD19 would remain in vivo, with as yet unclear consequences for the modulation of BCR signaling 75 77 121 . In vitro, anti-CD19 (HD37) lowers the threshold for anti-BCR (IgM)-induced human B-cell activation under certain conditions 142 , which could confer enhanced B-cell activation and even exacerbation of autoimmunity, while other reports using different protocols reported an inhibition of BCR signaling by anti-CD19 antibody. The above notions remain a theoretical possibility since overactivation of B cells during anti-CD19 treatment has not so far been reported.

B-cell depletion also affects B cells with immuno-suppressive potential, which might explain some rare cases of autoimmunity that were reported during rituximab therapy 143 144 145 146 . In this regard, several studies in mice indicate an important role of B cells producing IL-10 in controlling autoimmunity 147 . In vitro generated IL-10-producing B cells express CD19 148 149 and were therefore subject to CD19-directed depletion. The phenotype and the importance of human bona fide IL-10-secreting B cells in vivo remains unknown, however, and there is no clear evidence for impaired control of immune responses after either anti-CD19 or anti-CD20 therapy in humans.

The estimated immunological side effects of envisaged B-cell depletion using CD19-directed approaches mostly resemble the experiences from rituximab treatment. While additional putative depletion of pro-B cells leading to prolonged peripheral reconstitution and reductions of major PC subsets might lead to an enhanced risk for infections, CD19 therapy has the promise to improve efficacy compared with the more restricted B-cell targeting by anti-CD20 antibodies.

MEDI 545 is in clinical development by Medimmune Inc.