HLA Analysis in Autoimmune Encephalitis and Related Disorders: Part II

Autoimmune encephalitis (AE) are characterized by subacute onset of memory deficits, altered mental status or psychiatric symptoms, frequently associated with seizures, inflammatory cerebrospinal fluid and in cases with prominent limbic involvement, typical magnetic resonance imaging. Several autoantibodies (Abs) may be detected in AE, although its detection is not mandatory to establish a diagnosis. These Abs mainly recognize different synaptic and cell-surface proteins in the central nervous system, and are thought to be pathogenic as they alter the normal location or function of its antigens. Paraneoplastic neurological syndromes (PNS) are immune-mediated, remote complications of cancer. The clinical presentation is highly diverse, from central nervous system disorders (limbic encephalitis, cerebellar ataxia) to peripheral neuropathies and neuromuscular junction diseases. Two different kinds of Abs are associated with PNS: a first group known as onconeural Abs, which recognize intracellular antigens and are thought not to be pathogenic; and a second one whose targeted synaptic and cell-surface antigens shared with some non-paraneoplastic AE. The primary trigger of the immune response is unknown for most of AE. In addition to acquired susceptibility such as herpes simplex encephalitis, genetic predisposition may also be important in the pathogenesis of AE. Human leukocyte antigen (HLA) is the genetic factor most frequently associated with autoimmune diseases, due to its genetic complexity and key role in the adaptive immune response. Others and we already described the HLA haplotypes associated with three types different of AE: anti-leucine-rich glioma inactivated 1 (LGI1), anti-contactin-associated protein-like 2 (CASPR2), and anti-glutamic acid decarboxylase (GAD). Nevertheless, the genetic predisposition of many other AE has not been investigated yet. Cancer is considered as the trigger of the immune response that lead to PNS development, as the neural antigens recognized by the onconeural Abs are also expressed by tumor cells. Nevertheless, it is still unknown why some patients develop PNS and others do not, even if they present the same histological type of tumor, suggesting that some particular, maybe genetic, characteristics of the patients may play a role in this susceptibility. Furthermore, there is already evidence that, for those neurological diseases that may appear either as PNS or as non-paraneoplastic autoimmune disorder (i.e. Lambert-Eaton myasthenic syndrome), HLA profiles are not the same.