Comment on: PML in patients with systemic lupus erythematosus: a systematic literature review

Sir, We read with interest the report ‘Progressive multifocal leukoencephalopathy in patients with systemic lupus erythematosus: a systematic literature review’ and wish to emphasize that lymphopenia, which may be the most important risk factor for progressive multifocal leukoencephalopathy (PML), failed to be mentioned. In their thorough literature review, Henegar et al. 1 found that most systemic lupus erythematosus (SLE) patients (32 out of 35) were on immunosuppressant therapy (IST) at the time of PML diagnosis. Even though the IST type was discriminated, individual duration and effect of IST on the immune system could not be analysed. A recent case report from our units was associated with a fatal outcome. 2 The patient had severe CD4 þ T cell lymphopenia. Our review of the literature revealed that information about lymphocyte counts was scarce, but when available lymphopenia was invariably present in SLE patients with PML. PML was initially described in patients subjected to chemotherapy 3 and is a feared complication of idiopathic CD4 þ T cell lymphopenia. 4 Altogether, such compelling evidence leads us to believe that in order to prevent PML in SLE, lymphocyte counts should be frequently monitored and IST adjusted in order to avoid severe lymphopenia. Current recommendations support maintaining a total count above 1000 � 10 6 /L, 5 but in current practice we believe the management of lymphopenia in SLE remains an unmet need.