The combined detection of CD10, BCL6, PD-1 and CXCL13 has high specificity and sensitivity for the differential diagnosis of AITL. PD-1 and CXCL13 are more sensitive, superior diagnostic markers for AITL than CD10 and BCL6. Currently, T(FH) cell markers are the only available markers that show high specificity for AITL. LVPTCL, NOS and/or FVPTCL, NOS may also arise from T(FH) cells and fall within the spectrum of AITL.

The proton-coupled folate transporter (PCFT, SLC46A1) is required for folate intestinal absorption and transport across the choroid plexus. Recent work has identified a F392V mutation causing hereditary folate malabsorption. However, the residue properties responsible for this loss of function remains unknown. Using site-directed mutagenesis, we observed complete loss-of-function with charged (Lys, Asp, Glu) and polar (Thr, Ser, Gln) Phe392 substitutions and minimal function with some neutral substitutions; however, F392M retained full function. Using the substituted-cysteine accessibility method (with MTSEA-biotin labeling), Phe392 mutations causing loss of function, while preserving membrane expression and trafficking, also resulted in loss of accessibility of the substituted cysteine in P314C-PCFT located within the aqueous translocation pathway. F392V function and accessibility of the P314C cysteine were restored by insertion of a G305L (suppressor) mutation. A S196L mutation localized in proximity to Gly305 by homology modeling was inactive. However, when inserted into the inactive F392V scaffold, function was restored (mutually compensatory mutations) as was accessibility of the P314C cysteine residue. Reduced function, documented with F392H PCFT was due to a 15-fold decrease in methotrexate influx Vmax, accompanied by a decreased influx Kt (4.5-fold) and Ki (3-fold). The data indicate that Phe392 is required for rapid oscillation of the carrier among its conformational states and suggest that this is achieved by dampening affinity of the protein for its folate substrates. F392V, and other inactivating Phe392 PCFT mutations, lock the protein in its inward-open conformation. Reach (length) and hydrophobicity of Phe392 appear to be features required for full activity.