If you have it available, miltenyi have a gentle macs kit that can be used for isolating lymphocytes from lungs. I have used this for tumours and it works very well (link attached). If not, im sure there are protocols in the literature using similar mixtures of collagenases etc. 

http://www.miltenyibiotec.com/en/products-and-services/macs-sample-preparation/sample-dissociation/tissue-dissociation-kits/lung-dissociation-kit-mouse.aspx

We also use the gentleMACS machine and find it quite effective.  However, any lung samples can be minced finely (

If you have access to the animal before the lungs are taken out, it is possible to first inject anti-CD45 in one colour (e.g. red) iv before you sacrifice. After you sacrifice and isolate the lymphocytes, label the cells with anti-CD45 in another colour (e.g. green). The circulating lymphocytes will be red and green, whereas the tissue-resident lymphocytes will be green only. The linked paper takes this approach but with CD8.

If this is too difficult, I agree with Stephen that good perfusion/washing is key.

http://www.nature.com/ni/journal/v14/n3/full/ni.2525.html

• The major surface marker of cytotoxic T cells is CD8, also known as killer T cells.
• The major surface marker of helper T cells is CD4.
• Memory T cells consist of both CD4 and CD8 T cells that can rapidly acquire effector functions to kill infected cells and/or secrete inflammatory cytokines that inhibit replication of the pathogen.
• There are many classes of regulatory / suppressor T cells, including CD25 and CD4 T cells. They can inhibit T cells and B cells to regulate and control the immune response and maintain immune self-stability. http://www.creative-diagnostics.com/t-cell-differentiation.htm