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# Mechanisms of immune tolerance breakdown in paraneoplastic neurological syndromes

E. Peter, P. Dumez, J. Honnorat, V. Desestret

► To cite this version:

E. Peter, P. Dumez, J. Honnorat, V. Desestret. Mechanisms of immune tolerance breakdown in paraneoplastic neurological syndromes. 2024, pp.931-939. 10.1016/j.neurol.2024.08.002. hal-04986382

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General review

# Mechanisms of immune tolerance breakdown in paraneoplastic neurological syndromes

<figure><img src="image_3.png" /></figure>

E. Peter<sup>a,b,c</sup>, P. Dumez<sup>a,b,c</sup>, J. Honnorat<sup>a,b,c</sup>, V. Desestret<sup>a,b,c,*</sup>

INFO ARTICLE

Article history:
Received 2 June 2024
Received in revised form
1st August 2024
Accepted 28 August 2024
Available online 19 September 2024

Keywords:
Paraneoplastic neurological
syndrome
Antitumor immunity
Immune tolerance breakdown
Onconeural antigen

A B S T R A C T

Paraneoplastic neurological syndromes (PNS) are rare autoimmune disorders triggered by the presence of a cancer. The autoimmunity is herein directed against proteins expressed both in the tumor and in the nervous system, namely the onconeural antigens, against which are directed specific autoantibodies, each of them characterizing a neurological syndrome. The mechanisms of the immune tolerance breakdown in PNS leading to the production of specific autoantibodies directed against the nervous system and leading to the immune attack begins to be explained. Each syndrome is associated with a specific histo-molecular subtype of tumor suggesting a link between the PNS genesis and oncogenesis. The expression of the onconeural antigen by these tumors is insufficient to explain the immune tolerance breakdown. In some PNS tumors, alterations of the antigen have been identified: mutations, gene copy number variation and overexpression of transcript and protein. But in others PNS, no such molecular alterations of the onconeural antigens have been demons-trated. In these cases, other mechanisms of neoantigen generation that may be involved remain to be deciphered. Cancer outcomes of PNS tumors are also characterized by the high frequency of lymph node metastasis at diagnosis. At the primary tumor site, the antitumor immune reaction seems to be particularly intense and characterized by a prominence of B-cell and Ig-secreting plasma cells that may generate the autoantibody secretion. The immune control mechanisms leading to such organization of the immune attack are not known to date. Renewed research efforts are thus needed to better understand the mecha-nism of immune tolerance breakdown in each PNS and determine potential targets to meet the therapeutic challenges posed by these rare disorders.

© 2024 The Author(s). Published by Elsevier Masson SAS. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

<footnote><sup>a</sup>Inserm U1314/UMR CNRS5284, SynatAc Team, MeLis Institute, Lyon, France</footnote>

<footnote><sup>b</sup>French Reference Center on Paraneoplastic Neurological Syndromes, Hospices Civils de Lyon, Lyon, France</footnote>

<footnote><sup>c</sup>University of Lyon, Université Claude-Bernard Lyon 1, Lyon, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: virginie.desestret@chu-lyon.fr (V. Desestret).</footnote>

<footnote>https://doi.org/10.1016/j.neurol.2024.08.002</footnote>

<footnote>0035-3787/© 2024 The Author(s). Published by Elsevier Masson SAS. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>
## Abbreviations

<table><tr><td>CDR2</td><td>cerebellar degeneration related protein 2</td></tr><tr><td>CNS</td><td>central nervous system</td></tr><tr><td>DNER</td><td>delta and notch-like epidermal growth factor related receptor</td></tr><tr><td>ELAVL4</td><td>embryonic lethal abnormal vision-like protein 4</td></tr><tr><td>GABABR</td><td>gamma-aminobutyric acid receptor</td></tr><tr><td>HER2</td><td>human epidermal growth factor receptor 2</td></tr><tr><td>HL</td><td>Hodgkin's lymphoma</td></tr><tr><td>HLA</td><td>human leukocyte antigen</td></tr><tr><td>ICI</td><td>Immune checkpoint inhibitors</td></tr><tr><td>KHLH11</td><td>Kelch-like protein 11</td></tr><tr><td>MHC</td><td>major histocompatibility complex</td></tr><tr><td>NMDAR</td><td>N-methyl-d-aspartate receptor</td></tr><tr><td>NOVA</td><td>neuro-oncological ventral antigen</td></tr><tr><td>PNS</td><td>paraneoplastic neurological syndrome</td></tr><tr><td>RGS8</td><td>regulator of G protein signaling 8</td></tr><tr><td>SCLC</td><td>small-cell lung carcinoma</td></tr><tr><td>THSD7A</td><td>thrombospondin type I domain containing 7A</td></tr></table>

## 1. Introduction

Paraneoplastic neurological syndromes (PNS) are rare autoimmune disorders characterized by the presence of a cancer, affecting an otherwise immune-privileged site: the nervous system. Satellite of the antitumor immune response, the B-cell response generates a secretion of autoantibodies that are directed against a protein expressed both in the tumor and in the nervous system, the so-called onconeural antigen [1] As a result, the immune tolerance breakdown led by this cross-reaction supports a potential efficient local control of the tumor, but is also responsible for collateral damages in the brain, where the onconeural protein are naturally expressed [2,3]. The autoantibodies associated with PNS are powerful biomarkers of cancer and each subtype of autoantibodies is specifically associated with particular tumors and particular neurological syndromes (e.g., anti-Yo autoantibodies are specific to subacute cerebellar ataxia and gynecological malignancies [4]). In that respect, onconeural autoantibodies became the cornerstone of PNS diagnosis [5]. Moreover,

antibodies types are clues reflecting the probability of an associated cancer, “high-risk antibodies” categories being associated with a cancer in > 70% of patients [5]. Nonetheless, certain high-risk antibodies may be detected in asymptomatic patients with a cancer [6,7] and are not sufficient per se to trigger a full-blown clinical picture. While cancer prevalence is high in the general population, very few patients develop PNS and the reasons underlying the extreme rarity of these disorders remain elusive. This observation raises several questions: What are the triggering factors of autoimmunity in PNS? What specific characteristics of the tumor or its host allow such an immune reaction to occur? Where and how does the immune tolerance breakdown occur? This review will highlight the current evidence supporting an approach to answers to these questions.

## 2. Tumor specificities

### 2.1. Each PNS characterized by a high-risk autoantibody is associated with particular types of cancers

The selective association of certain primary tumors with each high-risk autoantibody has been described early in PNS and suggests a strong pathophysiological link between the tumor, the tissue from which the tumor is derived, and the PNS [4]. Conversely, numerous types of cancers are virtually never associated with paraneoplastic syndromes (e.g., skin cancers; epidermoid tumors such as ear-nose-throat, anus, vagina; gliomas; liver cancers) [4]. Classical high-risk autoantibodies and their associated cancers are described in Table 1. Already, it appears that the type of cancer is not in itself sufficient to predict the subtype of associated PNS as similar tumors can be observed with multiples autoantibodies (e.g. anti-Yo and anti-Ri autoantibodies are both associated with breast cancers [8,9]) and most antibodies are associated with several cancer types (e.g. anti-Ri can be associated with both breast and lung carcinoma [9]). Hence, not all tumors can be associated with a PNS whereas some cancer types are major providers of autoimmunity such as breast cancers and small-cell lung carcinoma (SCLC). These latter are nonetheless the two most prevalent malignancies worldwide [10] but are only exceptionally associated with PNS (less than 0.01%). Plus, current knowledge in oncology has, in most cancers, has enabled

<table><caption>Table 1 – Cancer association with high-risk autoantibodies.</caption><thead><tr><th>Autoantibody</th><th>Main cancer associations</th><th>Rare cancer associations</th></tr></thead><tbody><tr><td>Hu</td><td>Small-cell lung carcinoma (SCLC)</td><td>Non-small-cell lung carcinoma (NSCLC), other neuroendocrine tumors, pediatric neuroblastoma</td></tr><tr><td>CV2/CRMP5</td><td>SCLC, thymoma</td><td></td></tr><tr><td>SOX1</td><td>SCLC</td><td></td></tr><tr><td>PCA2</td><td>SCLC, NSCLC, breast cancer</td><td></td></tr><tr><td>Amphiphysin</td><td>SCLC</td><td>Breast cancer</td></tr><tr><td>Ri</td><td>SCLC, breast cancer</td><td></td></tr><tr><td>Yo</td><td>Ovarian cancer, breast cancer</td><td>Uterine cancer, cervix cancer</td></tr><tr><td>Ma2</td><td>Testicular seminoma, SCLC</td><td></td></tr><tr><td>DNER</td><td>Hodgkin's lymphoma</td><td>Anaplastic large T cell lymphoma</td></tr><tr><td>KLHL11</td><td>Testicular cancer</td><td></td></tr><tr><td>TRIM9/TRIM67</td><td>Lung adenocarcinoma, SCLC</td><td>Breast cancer, melanoma</td></tr></tbody></table>
classification of tumors into more subtle and homogeneous histomolecular subtypes, and study of their association with PNS has refined our understanding of the association between tumors and autoimmunity.

## 2.2. Selective association between tumor histomolecular subtypes and PNS suggests a link between oncogenesis and the immune tolerance breakdown

A major step in the understanding of the interaction between tumor oncogenesis and PNS occurrence has been the description of the association of anti-Yo PNS with a very specific histomolecular subtype of breast cancer, overexpressing human epidermal growth factor receptor 2 (HER2) but not the hormone receptors (namely, HER2-driven breast cancer) [3,11]. HER2-driven breast cancers represent less than 3.2% of all breast cancers diagnosed per year [12]. Importantly, a recent study showed that Ri-PNS-associated breast cancers are radically different from other cancers, as they do not express HER2 but overexpress the hormone receptors (also known as luminal B breast cancers that represent less than 15% of all the breast cancers diagnosed per year [13]. Similarly, ovarian cancers associated with anti-Yo PNS do not express HER2 [14]. Overall, HER2 overexpression seems to be a key element of PNS pathophysiology of anti-Yo PNS only in breast cancer and not in ovarian cancer. This suggests that, beyond cancer type, it is in some oncogenesis specificities that triggering factors of PNS auto-immunity could reside.

Similar conclusions could be drawn from the observations of anti-regulator of G protein signaling 8 (RGS8) [15] as well as anti-delta and notch-like epidermal growth factor related receptor (DNER) [16] PNS, that are both associated with Hodgkin's lymphoma (HL), but each with different subtypes according to World Health Organization classification [17]. Indeed, while anti-DNER is associated with nodular sclerosis classical HL [11], anti-RGS8 arises in a very minoritarian subtype called nodular lymphocyte-predominant HL (unpublished results).

Nonetheless, these histomolecular tumor subtypes are not exceptional on a population scale but only a very few proportions will develop a PNS. So, a particular subtype of tumor is necessary to develop a PNS; but this is not sufficient, and it is likely that other molecular specificities of PNS tumors, in particular of the onconeural antigen, may participate in triggering autoimmunity.

# 3. Onconeural antigens specificities

## 3.1. Tumor expression of the onconeural antigen is not sufficient for PNS triggering

The dogma of an ectopic expression of a neural antigen in the tumor as the main trigger of PNS has long been taken for granted [1]. In this respect, anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis associated with ovarian teratomas is a caricatural example of an abdominal location of neural tissue expressing the onconeural antigen targeted by the immune reaction [18-20]. Nevertheless, recent studies have

allowed to request this statement. First, the antigen can be naturally expressed in the native tissue from which derives the tumor, for instance Ri (neuro-oncological ventral antigen [NOVA] 1 and 2) in normal breast tissue. The expression of the antigen in a tumor deriving from a tissue in which it is physiologically expressed can therefore be associated with a paraneoplastic immune reaction [21]. Conversely, the antigen can be expressed in a tumor type irrespective of the association with a PNS, as it is described with embryonic lethal abnormal vision-like protein 4 (ELAVL4) and SCLC [22]. Third, the expression of an antigen classically associated with PNS can be found in different cancers, whereas not associated with a PNS. Cerebellar degeneration-related protein 2 (CDR2), one of the onconeural antigens of anti-Yo syndrome, is regularly expressed in papillary renal cell carcinoma [23], a neoplasm that is virtually never associated with anti-Yo syndrome [24]. Hence, though probably pivotal in most PNS, tumor expression of a neural antigen does not appear to be the only factor triggering the immune tolerance breakdown. The immunogenicity of the onconeural antigen may reside in other characteristics of the antigens.

## 3.2. Overexpression of the onconeural antigens is involved in certain PNS

Recent data in anti-Yo associated breast and ovarian cancers tend to suggest that an excessive expression of the antigen could per se be the trigger of immune tolerance breakdown, as described in antithrombospondin type I domain containing 7A (THSD7A) paraneoplastic membranous nephropathy [25]. This overexpression could be linked to the amplification of the antigen's gene locus or strong gains that would lead to excessive transcription of the onconeural antigen and thus overexpression of the protein in tumor cells, as seen in anti-Yo PNS (Fig. 1) [11,14]. However, neither the amplification nor the overexpression of the antigens are found in Hu- [22], GABABR- and Ri-PNS (unpublished results). Alternatively, other scenarios may occur, as in the case of paraneoplastic anti-gamma-aminobutyric acid receptor (GABABR) encephalitis associated with SCLC. Herein, it is one of the main GABABR intracellular interactors, KCTD16 (for potassium channel tetramerization domain containing 16) that displays a significant gain in copy number and transcript overexpression, while such modifications do not occur for the receptor itself. Noteworthy, the paraneoplastic forms of anti-GABABR encephalitis were described to be associated with anti-KCTD16 autoantibodies in patients' sera and cerebrospinal fluid, adding credit to the implication of this protein in the pathophysiology of this PNS [26].

## 3.3. Mutations of the onconeural antigen could sustain its recognition as non-self

Somatic mutations in tumors have also been shown to trigger the antitumor immunity via the creation of neoantigens [27]. Indeed, a high frequency of mutations of the Yo genes (CDR2L and its paralog CDR2) altering the subsequent protein sequence (mis-sense or stop-gain mutations) was found in patients with PNS and associated ovarian [14] or breast carcinomas [11] (in 65% and 62% of patients respectively,
<figure><img src="image_4.png" /><figcaption>**Fig. 1** – Summary of onconeural antigen alterations that could drive its specific immunogenicity, based on the examples of anti-Yo syndrome associated with breast or ovarian cancer, anti-Hu and anti-GababR associated with SCLC and anti-Ri associated with breast cancer. Anti-Yo syndrome (upper part of the figure): CDR2L, the onconeural antigen, is overexpressed in tumor cells (green protein in the tumor cells in the upper left corner). A focus in tumor cells' nuclei allows to describe several genetic alterations of the antigen: mutations of its sequence and amplifications of the gene's locus with potential consequence on neoantigen overexpression. Anti-GabaBR syndrome (bottom left corner): overexpression of KCTD16, one of the main intracellular interactors of the target receptor. Anti-Hu syndrome (bottom center): putative mechanisms of immunogenicity is post-translational modification of the protein sequence generating neoepitopes, no gene alteration or overexpression of the antigen is described. Anti-Ri syndrome (bottom right corner): possible implication of alternative splicing of the antigen resulting in an immunogenic isoform. NB: putative mechanisms are marked with an “?”.</figcaption></figure>

whereas control cohorts from the COSMIC database harbored such mutations in less than 0.2% of cases for both cancers [28]). In one anti-Yo patient with ovarian carcinoma, a missense mutation detected in the tumor was predicted to increase the affinity for Yo onconeural antigens of at least one MHC-class-I allele [14]. Other types of mutations of debatable relevance as to their immunogenicity such as frameshift mutations were also described more frequently on Yo genes in ovarian and breast carcinomas of anti-Yo PNS patients that could suggest that the genes encoding the corresponding antigens are tumor mutational hotspots. Overall, in anti-Yo PNS, the mutations of the antigens could be the reason for their recognition as non-self and the initiation of the immune reaction (Fig. 1). Conversely, in some PNS, no mutations of the antigens are found (see Table 2), reflecting the inconsistency of this pathophysiological mechanism in the different syndromes and the need for renewed research efforts to understand the triggers of immune tolerance breakdown in these syndromes.

### 3.4. Other mechanisms of immunization might be implicated in some PNS

Hu- and Ri-PNS are two examples of PNS in which neither genetic alterations nor overexpression of the antigens in PNS tumors compared to controls are described [22]. Nevertheless, in both syndromes, the associated tumors display an important expression of the antigen even when they are not associated with PNS (ELAVL4 is overexpressed in all SCLC [29] and NOVA1 and 2 are overexpressed in all luminal breast cancers (data from the cancer genomic atlas TCGA)), thus raising the question of other potential mechanisms altering the onconeural antigen immunogenicity. First, post-transcriptional modifications of the onconeural antigens, which was first described in rheumatoid arthritis and peptide citrullination [30-32], may occur and should be investigated in PNS (Fig. 1B). Recently, a study described the role of isospartylation in neoantigenicity of the Hu antigen (embryonic lethal abnormal vision-like protein 4 or ELAVL4 protein) in
<table><caption>Table 2 – Summary of the oncological specificities and antigen's characteristics for each PNS.</caption><thead><tr><th rowspan="2">Syndrome</th><th colspan="3">Cancer characteristics</th><th colspan="6">Antigen characteristics</th></tr><tr><th>Cancer</th><th>Histomolecular subtype</th><th>Metastasis at diagnosis</th><th>Chromosomal abnormalities</th><th>Mutation</th><th>Gene copy number variation</th><th>Transcript overexpression</th><th>Protein overexpression</th><th>Interactor overexpression</th></tr></thead><tbody><tr><td>Anti-Yo</td><td>Ovary</td><td>High grade serous carcinoma</td><td>66% of cases</td><td>17q amplification</td><td>60% of cases</td><td>70% gain (&lt; 6 copies)</td><td>NA</td><td>Yes</td><td>No</td></tr><tr><td></td><td>Breast</td><td>Inflammatory HER2+ ER- PR- HER2-driven Inflammatory HER2- ER+ PR+ Luminal B</td><td>80% of cases</td><td>HER2/CDR2L amplification</td><td>60% of cases</td><td>70% amplification (&gt; 6 copies)</td><td>Yes</td><td>Yes</td><td>No</td></tr><tr><td>Anti-Ri</td><td>Breast</td><td>Inflammatory HER2+ ER+ PR+ Luminal B</td><td>65% of cases</td><td></td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>Anti-Hu</td><td>SCLC</td><td>Inflammatory</td><td>95% of cases</td><td></td><td>No</td><td>No</td><td>No</td><td>No</td><td>No</td></tr><tr><td>Anti-GababR</td><td>SCLC</td><td>Inflammatory</td><td>95% of cases</td><td></td><td>No</td><td>No</td><td>No</td><td>No</td><td>Yes (KCTD16)</td></tr><tr><td>Anti-DNER</td><td>Hodgkin</td><td>Sclero nodular classical</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>Yes</td><td>No</td></tr><tr><td>Anti-RGS8</td><td>Hodgkin</td><td>Nodular lymphocyte predominant</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>Yes</td><td>No</td></tr><tr><td>Anti-KHLH11</td><td>Testicular cancer</td><td>Germ-cell tumor</td><td>78% of cases</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td></tr><tr><td>Anti-TRIM9/ TRIM67 [77]</td><td>Miscellaneous</td><td>NA</td><td>85% of cases</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>Yes</td><td>NA</td></tr></tbody></table>

SCLC [33], a promising lead that would need confirmatory studies.

Alternative splicing has also been suggested as a mechanism of immunogenic neoepitope generation in cancer [34,35], and may lead to the neoantigen creation responsible for PNS (Fig. 1C). This hypothesis is particularly interesting in the context of Hu- and Ri-PNS, in which targeted antigens, respectively ELAVL4 and NOVA1 and 2, are master regulators of neural genes splicing [36,37]. Herein, their dysregulation could lead to the creation of multiple isoforms acting as onconeural neoantigens, that would subsequently amplify the immune reaction.

Then, the possibility of multiples neoantigens has long been a question in the field of PNS. The clinical heterogeneity of certain syndromes (mainly anti-Hu and anti-Ri) has raised early on the hypothesis of a multi-target immune reaction in some of them. There is no apparent link between the recognized epitope on the antigen and the clinical phenotype [38]. This observation reinforced the hypothesis of the implication of multiples antigens, and further assessment with phage immunoprecipitation-sequencing (PhIP-Seq) analysis would be needed on PNS cohorts to answer this question [38-40].

Overall, many different mechanisms of immune tolerance breakdown may lead to PNS, as summed up in Fig. 1, but this variety of *primum movens* contrasts with the homogeneity of the subsequent immune reaction across all PNS that is discussed below.

## 4. Antitumor immune response specificities in PNS

### 4.1. A specific humoral in situ antitumor immune response in PNS

Recent studies have identified the immune cells involved in the antitumor immune response in the primary tumor site. Both in ovarian and breast carcinomas associated with anti-Yo PNS, similar observations of an intense immune response, with an unusual proportion of B-cell and IgG-producing plasma cells are reported [11,14]. This suggests that the autoantibody production takes place at the tumor site, as already demonstrated in anti-NMDAR-associated teratomas [19,20,41-44]. This humoral polarization of the antitumor immune response is quite an original characteristic of PNS-associated antitumor immunity, since these reactions are usually T-cell mediated in the same gynecological cancers without PNS [45,46]. Last, a noteworthy proportion of tumors associated with PNS present tertiary lymphoid structure in close contact with tumor cells, with an active germinal center that probably participates in the expansion of the immune reaction and orchestrates the in situ immune attack [11,14,20]. The role of lymphoid structures seems pivotal in PNS pathophysiology both at the tumor microenvironment level with the aforementioned tertiary lymphoid structure but also at a macroscopical level with the implication of secondary lymphoid structures.
## 4.2. High frequency of immune control of the primary tumor site contrasting with frequent lymphatic dissemination

In the context of PNS, the antitumor immune reaction sometimes has a remarkable efficiency, as shown by the unusually high frequency of clearance of the primary tumor site: in anti-KLHL-11 seminomas (the so-called "burn-out tumors") [47], but also in anti-Yo [11] and anti-Ri (unpublished results) breast cancers. In patients with anti-Hu PNS, a primary lung tumor is very rare and most of the time only mediastinal lymph node metastasis are identified (Macarena et al., Brain communications, 2023). Conversely, in some cancer types such as ovarian tumors, the immune reaction, though more important than in non-PNS tumors does not seem to successfully control the primary tumor site [14]. However, irrespective of the quality of tumor control at the primary site, most PNS-associated cancers share a common propension for early metastatic dissemination.

Indeed, the high frequency of lymph node metastasis at the time of PNS diagnosis has been described in several syndromes: anti-Kelch-like protein 11 (KLHL-11)-associated seminomas [47], anti-Yo-associated breast [11] or ovarian (unpublished results) cancers and anti-Ri-associated breast cancers (unpublished results), anti-Hu and anti-GABABR SCLC [48]. In addition, it is also not uncommon for a PNS to appear at time of nodal recurrence or progression discovery (unpublished results). These converging observations could allow to infer a role for lymphatic dissemination in the pathophysiology of PNS that is not yet understood.

## 4.3. Implication of lymphoid structures in PNS: the central hub?

One could hypothesize that, while the primary site might be exempt of alterations of the onconeural antigens, the secondary site might show different somatic mutations or protein alterations that could trigger the PNS. Alternatively, lymphatic dissemination could catalyze the antigen presentation, and thus act as a "second-hit" after the first activation in the primary site. The frequency of asynchrony between primary tumor appearance and anti-Yo clinical syndrome onset in ovarian carcinomas [14] also suggests a multi-step mechanism for PNS to appear. Overall, PNS pathophysiology might sometimes be a slow building process with specific turning points, one of which may be lymphatic dissemination of cancer cells. Whatever the role of lymphoid structures in antigen presentation, they also hold a role in the orchestration and execution of the immune reaction. The control mechanisms leading to such an organization and a polarization toward B-cell reaction and antibody production is not known to date but suggest an immune dysregulation that raises the question of potential factors related to the host that underpin such a reaction.

# 5. Host specificities

## 5.1. Individual propension to develop autoreactive lymphocytes

Some patients might have predisposition to autoimmunity that relies on the spontaneous production of autoreactive

lymphocytes, as described in other autoimmune diseases [49-52], that can in turn stimulate autoantibody production [50,53,54]. A similar scenario could explain the appearance of autoimmunity against a neural antigen without ectopic expression of the latter by a tumor. For instance, in anti-NMDAR encephalitis, a teratoma is present only in 20 to 25% of cases, the remaining 75-80% of patients developing typical anti-NMDAR encephalitis without expression of the antigen by a tumor [19,55]. Interestingly, the immune reaction differs in its nature between patients with and without teratoma in the sense that IgA autoantibodies seem to be specifically associated with the presence of a teratoma suggesting altogether different pathways for these two conditions [56]. Other autoantibodies, considered low or intermediate risk [5] are irregularly associated with a triggering tumor and one could hypothesize that the self-tolerance loss can occur without presentation of an onconeural antigen for these conditions.

## 5.2. Downstream defect in immune brakes

Several observations point toward a role for immune dysregulation in patients with PNS. First, it is frequent that patients or their relatives present with other systemic autoimmune diseases, suggesting a shared predisposition to self-tolerance loss [57-59]. Second, some patients with high-risk autoantibodies and a cancer might be purely asymptomatic as described in anti-Hu and SCLC [60] whereas others develop a fully blown clinical phenotype of PNS probably reflecting the interaction with a high-risk genotype or immune idiosyncrasy of the host. Third, patients with asymptomatic autoantibodies may develop a clinical phenotype of PNS after treatment with immune checkpoint inhibitors (ICI) while other patients will not [61-64]. This suggests that, more than predisposing factors, some patients may have idiosyncratic brakes to autoimmunity, as described in other conditions [65-68], that can be overrun when the immune system is artificially boosted by ICI treatments.

## 5.3. A non-HLA-related immune idiosyncratic predisposition

Human leukocyte antigen (HLA) is the most studied genetic factor associated with predisposition to autoimmune disease, and its relation to PNS is detailed in another review [57]. Briefly, no strong association has been demonstrated to date between classical PNS and specific HLA haplotypes [69,70] whereas such predispositions are often found in non-paraneoplastic autoimmune neurological disorders [71-76]. Further whole exome sequencing and gene-wide association study with large cohorts of patients would be necessary to screen for other genetic predispositions to develop PNS.

# 6. Conclusion

PNS is observed in patients with particular cancers. A specific oncogenesis seems to support the arising of a PNS when associated with onconeural antigens alterations. This alteration of the antigens can be mutations and amplification/
overexpression. Other putative mechanisms (alternative
splicing, post-translational modification) might be implicated
in certain syndromes. This combination of triggers finally
needs a host with a (spontaneously or ICI-induced) permissive
immune system to become clinically relevant. The lymphoid
structures could act in these conditions both as the immune
tolerance breakdown site but also as a hub relaying the
immune reaction to direct it against the tumor and the neural
self. Further studies are still needed to better understand the
mechanism of immune tolerance breakdown in each PNS and
determine potential targets to meet the therapeutic challenges
posed by these rare disorders.

Funding

This project has been developed within the BETPSY project, which is supported by a public grant overseen by the French national research agency (Agence nationale de la recherche, ANR), as part of the second "Investissements d'Avenir" program (reference ANR-18-RHUS-0012) and by the Institut National du Cancer (reference INCA_16723). EP received a grant from the Société Nationale Française de Médecine Interne (SNFMI).

Author contributions

Writing of the manuscript: EP, VD.

Revision of the manuscript: PD, JH, VD.

Disclosure of interest

The authors declare that they have no competing interest.

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